97. Interleaving String - Explanation

Description

You are given three strings s1, s2, and s3. Return true if s3 is formed by interleaving s1 and s2 together or false otherwise.

Interleaving two strings s and t is done by dividing s and t into n and m substrings respectively, where the following conditions are met

|n - m| <= 1, i.e. the difference between the number of substrings of s and t is at most 1.
s = s1 + s2 + ... + sn
t = t1 + t2 + ... + tm
Interleaving s and t is s1 + t1 + s2 + t2 + ... or t1 + s1 + t2 + s2 + ...

You may assume that s1, s2 and s3 consist of lowercase English letters.

Example 1:

Input: s1 = "aaaa", s2 = "bbbb", s3 = "aabbbbaa"

Output: true

Explanation: We can split s1 into ["aa", "aa"], s2 can remain as "bbbb" and s3 is formed by interleaving ["aa", "aa"] and "bbbb".

Example 2:

Input: s1 = "", s2 = "", s3 = ""

Output: true

Example 3:

Input: s1 = "abc", s2 = "xyz", s3 = "abxzcy"

Output: false

Explanation: We can't split s3 into ["ab", "xz", "cy"] as the order of characters is not maintained.

Constraints:

0 <= s1.length, s2.length <= 100
0 <= s3.length <= 200

Topics

String Dynamic Programming

Recommended Time & Space Complexity

You should aim for a solution as good or better than O(m * n) time and O(m * n) space, where m is the length of the string s1 and n is the length of the string s2.

Hint 1

If the sum of the characters in s1 and s2 does not equal s3, we return false. Think in terms of recursion and visualize it as a decision tree, where we explore different combinations of portions from both strings. Can you determine the possible decisions at each recursion step?

Hint 2

We recursively iterate through the strings using indices i, j, and k for s1, s2, and s3, respectively. At each step, we extend the current path in two directions based on whether the k-th character of s3 matches the current character of s1 or s2. If any path returns true, we immediately return true. If k goes out of bounds, it means we have successfully formed the interleaved string, so we return true.

Hint 3

This approach is exponential. Can you think of a way to optimize it? Since k depends on i and j, it can be treated as a constant, as we can derive k using i + j.

Hint 4

We can use memoization to cache the results of recursive calls and avoid redundant computations. Treating i and j as states, we can use a hash map or a 2D array to store the results.

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Prerequisites

Before attempting this problem, you should be comfortable with:

Recursion - Understanding how to break down string problems into smaller subproblems
Dynamic Programming (Memoization) - Caching recursive results to avoid redundant computation
2D Dynamic Programming - Building solutions using a 2D table where states depend on two string indices
String Manipulation - Working with string indices and character comparisons

1. Recursion

Intuition

This problem asks whether the string s3 can be formed by interleaving characters from s1 and s2, while keeping the relative order of characters from each string.

At any position in s3, we have at most two choices:

take the next character from s1
take the next character from s2

Using recursion, we try all valid ways of building s3 character by character.
The recursive function represents:
“Can we form s3 starting from index k, using characters from s1 starting at i and s2 starting at j?”

If we successfully consume all characters of s3 and also reach the end of both s1 and s2, then s3 is a valid interleaving.

Algorithm

Define a recursive function dfs(i, j, k):
- i is the current index in s1
- j is the current index in s2
- k is the current index in s3
If k reaches the end of s3:
- Return true only if both s1 and s2 are also fully used
If the next character of s1 matches s3[k]:
- Recurse by taking the character from s1
- If it returns true, stop and return true
If the next character of s2 matches s3[k]:
- Recurse by taking the character from s2
- If it returns true, stop and return true
If neither choice works:
- Return false
Start the recursion from indices (0, 0, 0)
Return the final result

class Solution:
    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:

        def dfs(i, j, k):
            if k == len(s3):
                return (i == len(s1)) and (j == len(s2))

            if i < len(s1) and s1[i] == s3[k]:
                if dfs(i + 1, j, k + 1):
                    return True

            if j < len(s2) and s2[j] == s3[k]:
                if dfs(i, j + 1, k + 1):
                    return True

            return False

        return dfs(0, 0, 0)

public class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {

        return dfs(0, 0, 0, s1, s2, s3);
    }

    private boolean dfs(int i, int j, int k, String s1, String s2, String s3) {
        if (k == s3.length()) {
            return (i == s1.length()) && (j == s2.length());
        }

        if (i < s1.length() && s1.charAt(i) == s3.charAt(k)) {
            if (dfs(i + 1, j, k + 1, s1, s2, s3)) {
                return true;
            }
        }

        if (j < s2.length() && s2.charAt(j) == s3.charAt(k)) {
            if (dfs(i, j + 1, k + 1, s1, s2, s3)) {
                return true;
            }
        }

        return false;
    }
}

class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        return dfs(0, 0, 0, s1, s2, s3);
    }

    bool dfs(int i, int j, int k, string& s1, string& s2, string& s3) {
        if (k == s3.length()) {
            return (i == s1.length()) && (j == s2.length());
        }

        if (i < s1.length() && s1[i] == s3[k]) {
            if (dfs(i + 1, j, k + 1, s1, s2, s3)) {
                return true;
            }
        }

        if (j < s2.length() && s2[j] == s3[k]) {
            if (dfs(i, j + 1, k + 1, s1, s2, s3)) {
                return true;
            }
        }

        return false;
    }
};

class Solution {
    /**
     * @param {string} s1
     * @param {string} s2
     * @param {string} s3
     * @return {boolean}
     */
    isInterleave(s1, s2, s3) {
        const dfs = (i, j, k) => {
            if (k === s3.length) {
                return i === s1.length && j === s2.length;
            }

            if (i < s1.length && s1[i] === s3[k]) {
                if (dfs(i + 1, j, k + 1)) {
                    return true;
                }
            }

            if (j < s2.length && s2[j] === s3[k]) {
                if (dfs(i, j + 1, k + 1)) {
                    return true;
                }
            }

            return false;
        };

        return dfs(0, 0, 0);
    }
}

public class Solution {
    public bool IsInterleave(string s1, string s2, string s3) {
        return dfs(0, 0, 0, s1, s2, s3);
    }

    private bool dfs(int i, int j, int k, string s1, string s2, string s3) {
        if (k == s3.Length) {
            return (i == s1.Length) && (j == s2.Length);
        }

        if (i < s1.Length && s1[i] == s3[k]) {
            if (dfs(i + 1, j, k + 1, s1, s2, s3)) {
                return true;
            }
        }

        if (j < s2.Length && s2[j] == s3[k]) {
            if (dfs(i, j + 1, k + 1, s1, s2, s3)) {
                return true;
            }
        }

        return false;
    }
}

func isInterleave(s1, s2, s3 string) bool {
    var dfs func(i, j, k int) bool
    dfs = func(i, j, k int) bool {
        if k == len(s3) {
            return i == len(s1) && j == len(s2)
        }

        if i < len(s1) && s1[i] == s3[k] {
            if dfs(i+1, j, k+1) {
                return true
            }
        }

        if j < len(s2) && s2[j] == s3[k] {
            if dfs(i, j+1, k+1) {
                return true
            }
        }

        return false
    }

    return dfs(0, 0, 0)
}

class Solution {
    fun isInterleave(s1: String, s2: String, s3: String): Boolean {
        fun dfs(i: Int, j: Int, k: Int): Boolean {
            if (k == s3.length) {
                return i == s1.length && j == s2.length
            }

            if (i < s1.length && s1[i] == s3[k]) {
                if (dfs(i + 1, j, k + 1)) {
                    return true
                }
            }

            if (j < s2.length && s2[j] == s3[k]) {
                if (dfs(i, j + 1, k + 1)) {
                    return true
                }
            }

            return false
        }

        return dfs(0, 0, 0)
    }
}

class Solution {
    func isInterleave(_ s1: String, _ s2: String, _ s3: String) -> Bool {
        let n1 = s1.count, n2 = s2.count, n3 = s3.count
        if n1 + n2 != n3 { return false }

        let s1 = Array(s1), s2 = Array(s2), s3 = Array(s3)

        func dfs(_ i: Int, _ j: Int, _ k: Int) -> Bool {
            if k == n3 {
                return i == n1 && j == n2
            }

            if i < n1 && s1[i] == s3[k] {
                if dfs(i + 1, j, k + 1) {
                    return true
                }
            }

            if j < n2 && s2[j] == s3[k] {
                if dfs(i, j + 1, k + 1) {
                    return true
                }
            }

            return false
        }

        return dfs(0, 0, 0)
    }
}

Time & Space Complexity

Time complexity: $O(2 ^ {m + n})$
Space complexity: $O(m + n)$

Where $m$ is the length of the string $s1$ and $n$ is the length of the string $s2$ .

2. Dynamic Programming (Top-Down)

Intuition

This problem asks whether the string s3 can be formed by interleaving characters from s1 and s2 while preserving the relative order of characters in both strings.

The recursive approach explores all possible interleavings, but many states repeat. To make it efficient, we use top-down dynamic programming (memoization).

A key observation is that the position in s3 is always determined by how many characters we have already taken from s1 and s2.
So the state can be defined using just:

index i in s1
index j in s2

The recursive function answers:
“Can we form the rest of s3 using s1[i:] and s2[j:]?”

Algorithm

First, check if the lengths of s1 and s2 add up to the length of s3:
- If not, return false immediately
Create a memoization map dp where:
- the key is (i, j)
- the value is whether s3[k:] can be formed from s1[i:] and s2[j:]
Define a recursive function dfs(i, j, k):
- i is the current index in s1
- j is the current index in s2
- k is the current index in s3
If k reaches the end of s3:
- Return true only if both s1 and s2 are fully used
If the state (i, j) is already in dp:
- Return the stored result
Try taking the next character from s1 if it matches s3[k]
If that does not work, try taking the next character from s2 if it matches s3[k]
Store the result in dp[(i, j)]
Start the recursion from (0, 0, 0)
Return the final result

class Solution:
    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:
        if len(s1) + len(s2) != len(s3):
            return False

        dp = {}
        def dfs(i, j, k):
            if k == len(s3):
                return (i == len(s1)) and (j == len(s2))
            if (i, j) in dp:
                return dp[(i, j)]

            res = False
            if i < len(s1) and s1[i] == s3[k]:
                res = dfs(i + 1, j, k + 1)
            if not res and j < len(s2) and s2[j] == s3[k]:
                res = dfs(i, j + 1, k + 1)

            dp[(i, j)] = res
            return res

        return dfs(0, 0, 0)

public class Solution {
    private Boolean[][] dp;

    public boolean isInterleave(String s1, String s2, String s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) return false;
        dp = new Boolean[m + 1][n + 1];
        return dfs(0, 0, 0, s1, s2, s3);
    }

    private boolean dfs(int i, int j, int k, String s1, String s2, String s3) {
        if (k == s3.length()) {
            return (i == s1.length()) && (j == s2.length());
        }
        if (dp[i][j] != null) {
            return dp[i][j];
        }

        boolean res = false;
        if (i < s1.length() && s1.charAt(i) == s3.charAt(k)) {
            res = dfs(i + 1, j, k + 1, s1, s2, s3);
        }
        if (!res && j < s2.length() && s2.charAt(j) == s3.charAt(k)) {
            res = dfs(i, j + 1, k + 1, s1, s2, s3);
        }

        dp[i][j] = res;
        return res;
    }
}

class Solution {
    vector<vector<int>> dp;

public:
    bool isInterleave(string s1, string s2, string s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) return false;
        dp = vector<vector<int>>(m + 1, vector<int>(n + 1, -1));
        return dfs(0, 0, 0, s1, s2, s3);
    }

    bool dfs(int i, int j, int k, string& s1, string& s2, string& s3) {
        if (k == s3.length()) {
            return (i == s1.length()) && (j == s2.length());
        }
        if (dp[i][j] != -1) {
            return dp[i][j];
        }

        bool res = false;
        if (i < s1.length() && s1[i] == s3[k]) {
            res = dfs(i + 1, j, k + 1, s1, s2, s3);
        }
        if (!res && j < s2.length() && s2[j] == s3[k]) {
            res = dfs(i, j + 1, k + 1, s1, s2, s3);
        }

        dp[i][j] = res;
        return res;
    }
};

class Solution {
    /**
     * @param {string} s1
     * @param {string} s2
     * @param {string} s3
     * @return {boolean}
     */
    isInterleave(s1, s2, s3) {
        const m = s1.length,
            n = s2.length;
        if (m + n !== s3.length) return false;

        const dp = Array.from({ length: m + 1 }, () => Array(n + 1).fill(-1));
        const dfs = (i, j, k) => {
            if (k === s3.length) {
                return i === s1.length && j === s2.length;
            }
            if (dp[i][j] !== -1) {
                return dp[i][j];
            }

            let res = false;
            if (i < s1.length && s1[i] === s3[k]) {
                res = dfs(i + 1, j, k + 1);
            }

            if (!res && j < s2.length && s2[j] === s3[k]) {
                res = dfs(i, j + 1, k + 1);
            }
            dp[i][j] = res;
            return res;
        };

        return dfs(0, 0, 0);
    }
}

public class Solution {
    private bool?[,] dp;

    public bool IsInterleave(string s1, string s2, string s3) {
        int m = s1.Length, n = s2.Length;
        if (m + n != s3.Length) return false;
        dp = new bool?[m + 1, n + 1];
        return dfs(0, 0, 0, s1, s2, s3);
    }

    private bool dfs(int i, int j, int k, string s1, string s2, string s3) {
        if (k == s3.Length) {
            return (i == s1.Length) && (j == s2.Length);
        }
        if (dp[i, j].HasValue) {
            return dp[i, j].Value;
        }

        bool res = false;
        if (i < s1.Length && s1[i] == s3[k]) {
            res = dfs(i + 1, j, k + 1, s1, s2, s3);
        }
        if (!res && j < s2.Length && s2[j] == s3[k]) {
            res = dfs(i, j + 1, k + 1, s1, s2, s3);
        }

        dp[i, j] = res;
        return res;
    }
}

class Solution {
    fun isInterleave(s1: String, s2: String, s3: String): Boolean {
        if (s1.length + s2.length != s3.length) {
            return false
        }

        val m = s1.length
        val n = s2.length
        val dp = Array(m + 1) { IntArray(n + 1) { -1 } }

        fun dfs(i: Int, j: Int, k: Int): Boolean {
            if (k == s3.length) {
                return i == m && j == n
            }

            if (dp[i][j] != -1) {
                return dp[i][j] == 1
            }

            var res = false
            if (i < s1.length && s1[i] == s3[k]) {
                res = dfs(i + 1, j, k + 1)
            }
            if (!res && j < s2.length && s2[j] == s3[k]) {
                res = dfs(i, j + 1, k + 1)
            }

            dp[i][j] = if (res) 1 else 0

            return res
        }

        return dfs(0, 0, 0)
    }
}

class Solution {
    func isInterleave(_ s1: String, _ s2: String, _ s3: String) -> Bool {
        let m = s1.count, n = s2.count, l = s3.count
        if m + n != l { return false }

        let s1 = Array(s1), s2 = Array(s2), s3 = Array(s3)
        var dp = Array(repeating: Array(repeating: nil as Bool?, count: n + 1), count: m + 1)

        func dfs(_ i: Int, _ j: Int, _ k: Int) -> Bool {
            if k == l {
                return i == m && j == n
            }
            if let cached = dp[i][j] {
                return cached
            }

            var res = false
            if i < m && s1[i] == s3[k] {
                res = dfs(i + 1, j, k + 1)
            }
            if !res && j < n && s2[j] == s3[k] {
                res = dfs(i, j + 1, k + 1)
            }

            dp[i][j] = res
            return res
        }

        return dfs(0, 0, 0)
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(m * n)$

Where $m$ is the length of the string $s1$ and $n$ is the length of the string $s2$ .

3. Dynamic Programming (Bottom-Up)

Intuition

We need to check whether the string s3 can be formed by interleaving s1 and s2, while keeping the relative order of characters from both strings.

Instead of recursion, we can solve this using bottom-up dynamic programming.
The idea is to determine, for every possible pair of positions (i, j), whether it is possible to form the suffix of s3 starting at position i + j using:

the substring s1[i:]
the substring s2[j:]

If either taking the next character from s1 or from s2 leads to a valid state, then the current state is also valid.

Algorithm

First, check if the lengths of s1 and s2 add up to the length of s3:
- If not, return false
Create a 2D DP table dp of size (len(s1) + 1) x (len(s2) + 1):
- dp[i][j] is true if s3[i + j:] can be formed using s1[i:] and s2[j:]
Initialize the base case:
- dp[len(s1)][len(s2)] = true because empty strings can form an empty string
Fill the table in reverse order (from bottom-right to top-left):
For each position (i, j):
- If the next character of s1 matches s3[i + j] and dp[i + 1][j] is true, then set dp[i][j] = true
- If the next character of s2 matches s3[i + j] and dp[i][j + 1] is true, then set dp[i][j] = true
After filling the table, the answer is stored in dp[0][0]
Return dp[0][0]

class Solution:
    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:
        if len(s1) + len(s2) != len(s3):
            return False

        dp = [[False] * (len(s2) + 1) for i in range(len(s1) + 1)]
        dp[len(s1)][len(s2)] = True

        for i in range(len(s1), -1, -1):
            for j in range(len(s2), -1, -1):
                if i < len(s1) and s1[i] == s3[i + j] and dp[i + 1][j]:
                    dp[i][j] = True
                if j < len(s2) and s2[j] == s3[i + j] and dp[i][j + 1]:
                    dp[i][j] = True
        return dp[0][0]

public class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) {
            return false;
        }

        boolean[][] dp = new boolean[m + 1][n + 1];
        dp[m][n] = true;

        for (int i = m; i >= 0; i--) {
            for (int j = n; j >= 0; j--) {
                if (i < m && s1.charAt(i) == s3.charAt(i + j) && dp[i + 1][j]) {
                    dp[i][j] = true;
                }
                if (j < n && s2.charAt(j) == s3.charAt(i + j) && dp[i][j + 1]) {
                    dp[i][j] = true;
                }
            }
        }
        return dp[0][0];
    }
}

class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) {
            return false;
        }

        vector<vector<bool>> dp(m + 1, vector<bool>(n + 1, false));
        dp[m][n] = true;

        for (int i = m; i >= 0; i--) {
            for (int j = n; j >= 0; j--) {
                if (i < m && s1[i] == s3[i + j] && dp[i + 1][j]) {
                    dp[i][j] = true;
                }
                if (j < n && s2[j] == s3[i + j] && dp[i][j + 1]) {
                    dp[i][j] = true;
                }
            }
        }
        return dp[0][0];
    }
};

class Solution {
    /**
     * @param {string} s1
     * @param {string} s2
     * @param {string} s3
     * @return {boolean}
     */
    isInterleave(s1, s2, s3) {
        let m = s1.length,
            n = s2.length;
        if (m + n !== s3.length) {
            return false;
        }

        const dp = Array.from({ length: m + 1 }, () =>
            Array(n + 1).fill(false),
        );
        dp[m][n] = true;

        for (let i = m; i >= 0; i--) {
            for (let j = n; j >= 0; j--) {
                if (i < m && s1[i] === s3[i + j] && dp[i + 1][j]) {
                    dp[i][j] = true;
                }
                if (j < n && s2[j] === s3[i + j] && dp[i][j + 1]) {
                    dp[i][j] = true;
                }
            }
        }
        return dp[0][0];
    }
}

public class Solution {
    public bool IsInterleave(string s1, string s2, string s3) {
        int m = s1.Length, n = s2.Length;
        if (m + n != s3.Length) {
            return false;
        }

        bool[,] dp = new bool[m + 1, n + 1];
        dp[m, n] = true;

        for (int i = m; i >= 0; i--) {
            for (int j = n; j >= 0; j--) {
                if (i < m && s1[i] == s3[i + j] && dp[i + 1, j]) {
                    dp[i, j] = true;
                }
                if (j < n && s2[j] == s3[i + j] && dp[i, j + 1]) {
                    dp[i, j] = true;
                }
            }
        }
        return dp[0, 0];
    }
}

func isInterleave(s1 string, s2 string, s3 string) bool {
    if len(s1) + len(s2) != len(s3) {
        return false
    }

    dp := make([][]bool, len(s1)+1)
    for i := range dp {
        dp[i] = make([]bool, len(s2)+1)
    }
    dp[len(s1)][len(s2)] = true

    for i := len(s1); i >= 0; i-- {
        for j := len(s2); j >= 0; j-- {
            if i < len(s1) && s1[i] == s3[i+j] && dp[i+1][j] {
                dp[i][j] = true
            }
            if j < len(s2) && s2[j] == s3[i+j] && dp[i][j+1] {
                dp[i][j] = true
            }
        }
    }

    return dp[0][0]
}

class Solution {
    fun isInterleave(s1: String, s2: String, s3: String): Boolean {
        if (s1.length + s2.length != s3.length) {
            return false
        }

        val dp = Array(s1.length + 1) { BooleanArray(s2.length + 1) }
        dp[s1.length][s2.length] = true

        for (i in s1.length downTo 0) {
            for (j in s2.length downTo 0) {
                if (i < s1.length && s1[i] == s3[i + j] && dp[i + 1][j]) {
                    dp[i][j] = true
                }
                if (j < s2.length && s2[j] == s3[i + j] && dp[i][j + 1]) {
                    dp[i][j] = true
                }
            }
        }

        return dp[0][0]
    }
}

class Solution {
    func isInterleave(_ s1: String, _ s2: String, _ s3: String) -> Bool {
        let m = s1.count, n = s2.count, l = s3.count
        if m + n != l { return false }

        let s1 = Array(s1), s2 = Array(s2), s3 = Array(s3)
        var dp = Array(repeating: Array(repeating: false, count: n + 1), count: m + 1)
        dp[m][n] = true

        for i in stride(from: m, through: 0, by: -1) {
            for j in stride(from: n, through: 0, by: -1) {
                if i < m && s1[i] == s3[i + j] && dp[i + 1][j] {
                    dp[i][j] = true
                }
                if j < n && s2[j] == s3[i + j] && dp[i][j + 1] {
                    dp[i][j] = true
                }
            }
        }
        return dp[0][0]
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(m * n)$

Where $m$ is the length of the string $s1$ and $n$ is the length of the string $s2$ .

4. Dynamic Programming (Space Optimized)

Intuition

We want to know if s3 can be built by interleaving s1 and s2 while keeping the order of characters from each string.

In the 2D DP solution, we used a table dp[i][j] to represent whether s3[i + j:] can be formed using s1[i:] and s2[j:].
But notice something important: to compute row i, we only need information from:

the row below (i + 1) and
the current row as we move across columns

So we do not need the full 2D table. We can compress it and keep only one row at a time, which reduces memory usage.

To make this even more efficient, we ensure that s2 is the longer string so the 1D array stays as small as possible.

Algorithm

Let m = len(s1) and n = len(s2). If m + n != len(s3), return false.
If s2 is shorter than s1, swap them so that s2 is always the longer string.
Create a 1D boolean array dp of size n + 1:
- dp[j] will represent the DP values from the "next row" (i.e., for i + 1)
Initialize the base case where both strings are fully used:
- set the last position to true
Iterate i from m down to 0:
- Create a new array nextDp for the current row
- If i == m, set nextDp[n] = true (empty suffixes match)
Iterate j from n down to 0:
- If we can take the next character from s1 (matches s3[i + j]) and dp[j] is true, set nextDp[j] = true
- If we can take the next character from s2 (matches s3[i + j]) and nextDp[j + 1] is true, set nextDp[j] = true
After finishing the row, assign dp = nextDp
The final answer will be dp[0], meaning we can form s3 starting from the beginning of both strings

class Solution:
    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:
        m, n = len(s1), len(s2)
        if m + n != len(s3):
            return False
        if n < m:
            s1, s2 = s2, s1
            m, n = n, m

        dp = [False for _ in range(n + 1)]
        dp[n] = True
        for i in range(m, -1, -1):
            nextDp = [False for _ in range(n + 1)]
            if i == m:
                nextDp[n] = True
            for j in range(n, -1, -1):
                if i < m and s1[i] == s3[i + j] and dp[j]:
                    nextDp[j] = True
                if j < n and s2[j] == s3[i + j] and nextDp[j + 1]:
                    nextDp[j] = True
            dp = nextDp
        return dp[0]

public class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) return false;
        if (n < m) {
            String temp = s1;
            s1 = s2;
            s2 = temp;
            int tempLength = m;
            m = n;
            n = tempLength;
        }

        boolean[] dp = new boolean[n + 1];
        dp[n] = true;
        for (int i = m; i >= 0; i--) {
            boolean[] nextDp = new boolean[n + 1];
            if (i == m) nextDp[n] = true;
            for (int j = n; j >= 0; j--) {
                if (i < m && s1.charAt(i) == s3.charAt(i + j) && dp[j]) {
                    nextDp[j] = true;
                }
                if (j < n && s2.charAt(j) == s3.charAt(i + j) && nextDp[j + 1]) {
                    nextDp[j] = true;
                }
            }
            dp = nextDp;
        }
        return dp[0];
    }
}

class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int m = s1.size(), n = s2.size();
        if (m + n != s3.size()) return false;
        if (n < m) {
            swap(s1, s2);
            swap(m, n);
        }

        vector<bool> dp(n + 1);
        dp[n] = true;
        for (int i = m; i >= 0; --i) {
            vector<bool> nextDp(n + 1);
            if (i == m) nextDp[n] = true;
            for (int j = n; j >= 0; --j) {
                if (i < m && s1[i] == s3[i + j] && dp[j]) {
                    nextDp[j] = true;
                }
                if (j < n && s2[j] == s3[i + j] && nextDp[j + 1]) {
                    nextDp[j] = true;
                }
            }
            dp = nextDp;
        }
        return dp[0];
    }
};

class Solution {
    /**
     * @param {string} s1
     * @param {string} s2
     * @param {string} s3
     * @return {boolean}
     */
    isInterleave(s1, s2, s3) {
        let m = s1.length,
            n = s2.length;
        if (m + n !== s3.length) return false;
        if (n < m) {
            [s1, s2] = [s2, s1];
            [m, n] = [n, m];
        }

        let dp = Array(n + 1).fill(false);
        dp[n] = true;
        for (let i = m; i >= 0; i--) {
            let nextDp = Array(n + 1).fill(false);
            if (i === m) nextDp[n] = true;
            for (let j = n; j >= 0; j--) {
                if (i < m && s1[i] === s3[i + j] && dp[j]) {
                    nextDp[j] = true;
                }
                if (j < n && s2[j] === s3[i + j] && nextDp[j + 1]) {
                    nextDp[j] = true;
                }
            }
            dp = nextDp;
        }
        return dp[0];
    }
}

public class Solution {
    public bool IsInterleave(string s1, string s2, string s3) {
        int m = s1.Length, n = s2.Length;
        if (m + n != s3.Length) return false;
        if (n < m) {
            var temp = s1;
            s1 = s2;
            s2 = temp;
            int tempLength = m;
            m = n;
            n = tempLength;
        }

        bool[] dp = new bool[n + 1];
        dp[n] = true;
        for (int i = m; i >= 0; i--) {
            bool[] nextDp = new bool[n + 1];
            if (i == m) nextDp[n] = true;
            for (int j = n; j >= 0; j--) {
                if (i < m && s1[i] == s3[i + j] && dp[j]) {
                    nextDp[j] = true;
                }
                if (j < n && s2[j] == s3[i + j] && nextDp[j + 1]) {
                    nextDp[j] = true;
                }
            }
            dp = nextDp;
        }
        return dp[0];
    }
}

func isInterleave(s1 string, s2 string, s3 string) bool {
    m, n := len(s1), len(s2)
    if m + n != len(s3) {
        return false
    }
    if n < m {
        s1, s2 = s2, s1
        m, n = n, m
    }

    dp := make([]bool, n+1)
    dp[n] = true
    for i := m; i >= 0; i-- {
        nextDp := make([]bool, n+1)
        if i == m {
            nextDp[n] = true
        }
        for j := n; j >= 0; j-- {
            if i < m && s1[i] == s3[i+j] && dp[j] {
                nextDp[j] = true
            }
            if j < n && s2[j] == s3[i+j] && nextDp[j+1] {
                nextDp[j] = true
            }
        }
        dp = nextDp
    }
    return dp[0]
}

class Solution {
    fun isInterleave(s1: String, s2: String, s3: String): Boolean {
        var s1 = s1
        var s2 = s2
        var m = s1.length
        var n = s2.length
        if (m + n != s3.length) {
            return false
        }
        if (n < m) {
            val temp = s1
            s1 = s2
            s2 = temp

            val temp1 = m
            m = n
            n = temp1
        }

        val dp = BooleanArray(n + 1)
        dp[n] = true
        for (i in m downTo 0) {
            val nextDp = BooleanArray(n + 1)
            if (i == m) nextDp[n] = true
            for (j in n downTo 0) {
                if (i < m && s1[i] == s3[i + j] && dp[j]) {
                    nextDp[j] = true
                }
                if (j < n && s2[j] == s3[i + j] && nextDp[j + 1]) {
                    nextDp[j] = true
                }
            }
            System.arraycopy(nextDp, 0, dp, 0, n + 1)
        }
        return dp[0]
    }
}

class Solution {
    func isInterleave(_ s1: String, _ s2: String, _ s3: String) -> Bool {
        var s1 = Array(s1), s2 = Array(s2), s3 = Array(s3)
        var m = s1.count, n = s2.count
        if m + n != s3.count { return false }
        if n < m {
            swap(&s1, &s2)
            swap(&m, &n)
        }

        var dp = Array(repeating: false, count: n + 1)
        dp[n] = true

        for i in stride(from: m, through: 0, by: -1) {
            var nextDp = Array(repeating: false, count: n + 1)
            if i == m {
                nextDp[n] = true
            }
            for j in stride(from: n, through: 0, by: -1) {
                if i < m && s1[i] == s3[i + j] && dp[j] {
                    nextDp[j] = true
                }
                if j < n && s2[j] == s3[i + j] && nextDp[j + 1] {
                    nextDp[j] = true
                }
            }
            dp = nextDp
        }
        return dp[0]
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(min(m, n))$

Where $m$ is the length of the string $s1$ and $n$ is the length of the string $s2$ .

5. Dynamic Programming (Optimal)

Intuition

We want to check if s3 can be formed by interleaving s1 and s2 while keeping the order of characters from both strings.

A common DP idea is:

at positions (i, j), we have used i characters from s1 and j characters from s2
so the next character we must match in s3 is at index i + j

From this state, we can move forward in two ways:

take s1[i] if it matches s3[i + j]
take s2[j] if it matches s3[i + j]

The 2D DP solution stores this for every (i, j), but we can do better:

each DP row only depends on the row below and the current row being built
so we can reuse a single 1D array
and instead of building a separate next array, we can update the 1D array in-place using one extra variable that tracks the “right neighbor” value

We also swap strings so that s2 is the longer one, keeping the DP array as small as possible.

Algorithm

Let m = len(s1) and n = len(s2).
If m + n != len(s3), return false immediately.
If s2 is shorter than s1, swap them so the DP array size becomes O(min(m, n)).
Create a boolean array dp of size n + 1:
- dp[j] represents whether s3[i + j:] can be formed using s1[i:] and s2[j:] for the current i
Initialize the base case:
- set dp[n] = true (when both suffixes are empty)
Iterate i from m down to 0:
- keep a variable nextDp that represents the value to the right (dp[j + 1]) for the current row
- initialize it as true only when i == m (bottom row base case)
Iterate j from n down to 0:
- compute whether the state (i, j) is valid:
  - it is valid if taking from s1 matches and the state below (dp[j]) was valid
  - or taking from s2 matches and the state to the right (nextDp) is valid
- write the result back into dp[j] (in-place update)
- update nextDp to the new dp[j] for the next iteration to the left
After all updates, dp[0] tells whether s3 can be formed starting from the beginning of both strings.
Return dp[0]

class Solution:
    def isInterleave(self, s1: str, s2: str, s3: str) -> bool:
        m, n = len(s1), len(s2)
        if m + n != len(s3):
            return False
        if n < m:
            s1, s2 = s2, s1
            m, n = n, m

        dp = [False for _ in range(n + 1)]
        dp[n] = True
        for i in range(m, -1, -1):
            nextDp = True if i == m else False
            for j in range(n, -1, -1):
                res = False if j < n else nextDp
                if i < m and s1[i] == s3[i + j] and dp[j]:
                    res = True
                if j < n and s2[j] == s3[i + j] and nextDp:
                    res = True
                dp[j] = res
                nextDp = dp[j]
        return dp[0]

public class Solution {
    public boolean isInterleave(String s1, String s2, String s3) {
        int m = s1.length(), n = s2.length();
        if (m + n != s3.length()) return false;
        if (n < m) {
            String temp = s1;
            s1 = s2;
            s2 = temp;
            int tempLen = m;
            m = n;
            n = tempLen;
        }

        boolean[] dp = new boolean[n + 1];
        dp[n] = true;
        for (int i = m; i >= 0; i--) {
            boolean nextDp = (i == m ? true : false);
            for (int j = n; j >= 0; j--) {
                boolean res = (j < n ? false : nextDp);
                if (i < m && s1.charAt(i) == s3.charAt(i + j) && dp[j]) {
                    res = true;
                }
                if (j < n && s2.charAt(j) == s3.charAt(i + j) && nextDp) {
                    res = true;
                }
                dp[j] = res;
                nextDp = dp[j];
            }
        }
        return dp[0];
    }
}

class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int m = s1.size(), n = s2.size();
        if (m + n != s3.size()) return false;
        if (n < m) {
            swap(s1, s2);
            swap(m, n);
        }

        vector<bool> dp(n + 1, false);
        dp[n] = true;
        for (int i = m; i >= 0; i--) {
            bool nextDp = (i == m ? true : false);
            for (int j = n; j >= 0; j--) {
                bool res = (j < n ? false : nextDp);
                if (i < m && s1[i] == s3[i + j] && dp[j]) {
                    res = true;
                }
                if (j < n && s2[j] == s3[i + j] && nextDp) {
                    res = true;
                }
                dp[j] = res;
                nextDp = dp[j];
            }
        }
        return dp[0];
    }
};

class Solution {
    /**
     * @param {string} s1
     * @param {string} s2
     * @param {string} s3
     * @return {boolean}
     */
    isInterleave(s1, s2, s3) {
        let m = s1.length,
            n = s2.length;
        if (m + n !== s3.length) return false;
        if (n < m) {
            [s1, s2] = [s2, s1];
            [m, n] = [n, m];
        }

        let dp = Array(n + 1).fill(false);
        dp[n] = true;
        for (let i = m; i >= 0; i--) {
            let nextDp = (i === m ? true : false);
            for (let j = n; j >= 0; j--) {
                let res = (j < n ? false : nextDp);
                if (i < m && s1[i] === s3[i + j] && dp[j]) {
                    res = true;
                }
                if (j < n && s2[j] === s3[i + j] && nextDp) {
                    res = true;
                }
                dp[j] = res;
                nextDp = dp[j];
            }
        }
        return dp[0];
    }
}

public class Solution {
    public bool IsInterleave(string s1, string s2, string s3) {
        int m = s1.Length, n = s2.Length;
        if (m + n != s3.Length) return false;
        if (n < m) {
            var temp = s1;
            s1 = s2;
            s2 = temp;
            int tempLen = m;
            m = n;
            n = tempLen;
        }

        bool[] dp = new bool[n + 1];
        dp[n] = true;
        for (int i = m; i >= 0; i--) {
            bool nextDp = (i == m ? true : false);
            for (int j = n; j >= 0; j--) {
                bool res = (j < n ? false : nextDp);
                if (i < m && s1[i] == s3[i + j] && dp[j]) {
                    res = true;
                }
                if (j < n && s2[j] == s3[i + j] && nextDp) {
                    res = true;
                }
                dp[j] = res;
                nextDp = dp[j];
            }
        }
        return dp[0];
    }
}

func isInterleave(s1, s2, s3 string) bool {
    m, n := len(s1), len(s2)
    if m+n != len(s3) {
        return false
    }
    if n < m {
        s1, s2 = s2, s1
        m, n = n, m
    }

    dp := make([]bool, n+1)
    dp[n] = true
    for i := m; i >= 0; i-- {
        nextDp := (i == m)
        for j := n; j >= 0; j-- {
            res := nextDp
            if j < n {
                res = false
            }
            if i < m && s1[i] == s3[i+j] && dp[j] {
                res = true
            }
            if j < n && s2[j] == s3[i+j] && nextDp {
                res = true
            }
            dp[j] = res
            nextDp = dp[j]
        }
    }
    return dp[0]
}

class Solution {
    fun isInterleave(s1: String, s2: String, s3: String): Boolean {
        var s1 = s1
        var s2 = s2
        var m = s1.length
        var n = s2.length
        if (m + n != s3.length) {
            return false
        }
        if (n < m) {
            val temp = s1
            s1 = s2
            s2 = temp

            val temp1 = m
            m = n
            n = temp1
        }

        val dp = BooleanArray(n + 1)
        dp[n] = true
        for (i in m downTo 0) {
            var nextDp = (i == m)
            for (j in n downTo 0) {
                var res = nextDp
                if (j < n) res = false
                if (i < m && s1[i] == s3[i + j] && dp[j]) {
                    res = true
                }
                if (j < n && s2[j] == s3[i + j] && nextDp) {
                    res = true
                }
                dp[j] = res
                nextDp = dp[j]
            }
        }
        return dp[0]
    }
}

class Solution {
    func isInterleave(_ s1: String, _ s2: String, _ s3: String) -> Bool {
        var s1 = Array(s1), s2 = Array(s2), s3 = Array(s3)
        var m = s1.count, n = s2.count
        if m + n != s3.count { return false }
        if n < m {
            swap(&s1, &s2)
            swap(&m, &n)
        }

        var dp = Array(repeating: false, count: n + 1)
        dp[n] = true

        for i in stride(from: m, through: 0, by: -1) {
            var nextDp = (i == m)
            for j in stride(from: n, through: 0, by: -1) {
                var res = nextDp
                if j < n {
                    res = false
                }
                if i < m && s1[i] == s3[i + j] && dp[j] {
                    res = true
                }
                if j < n && s2[j] == s3[i + j] && nextDp {
                    res = true
                }
                dp[j] = res
                nextDp = dp[j]
            }
        }
        return dp[0]
    }
}

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(min(m, n))$

Where $m$ is the length of the string $s1$ and $n$ is the length of the string $s2$ .

Common Pitfalls

Skipping the Length Check

A quick optimization that many solutions miss is checking if len(s1) + len(s2) == len(s3) upfront. If the lengths do not match, interleaving is impossible regardless of the characters. Skipping this check leads to unnecessary computation and can cause index out-of-bounds errors in some implementations.

Confusing Position Tracking with Three Indices

Since k = i + j always holds (where k is position in s3, i in s1, j in s2), you only need to track two indices. Some implementations incorrectly track all three independently, leading to inconsistent states or missed memoization opportunities. The key insight is that knowing positions in s1 and s2 uniquely determines the position in s3.

Greedy Character Matching

When both s1[i] and s2[j] match s3[k], you cannot greedily choose one over the other. Both branches must be explored. A common bug is to always prefer taking from s1 (or s2) when both match, which fails for cases like s1 = "a", s2 = "a", s3 = "aa" where either order works but a greedy approach might get stuck.