322. Coin Change - Explanation

Description

You are given an integer array coins representing coins of different denominations (e.g. 1 dollar, 5 dollars, etc) and an integer amount representing a target amount of money.

Return the fewest number of coins that you need to make up the exact target amount. If it is impossible to make up the amount, return -1.

You may assume that you have an unlimited number of each coin.

Example 1:

Input: coins = [1,5,10], amount = 12

Output: 3

Explanation: 12 = 10 + 1 + 1. Note that we do not have to use every kind coin available.

Example 2:

Input: coins = [2], amount = 3

Output: -1

Explanation: The amount of 3 cannot be made up with coins of 2.

Example 3:

Input: coins = [1], amount = 0

Output: 0

Explanation: Choosing 0 coins is a valid way to make up 0.

Constraints:

1 <= coins.length <= 10
1 <= coins[i] <= 2^31 - 1
0 <= amount <= 10000

Topics

Recommended Time & Space Complexity

You should aim for a solution with O(n * t) time and O(t) space, where n is the number of coins and t is the given amount.

Hint 1

Think of this problem in terms of recursion and try to visualize the decision tree, as there are multiple choices at each step. We start with the given amount. At each step of recursion, we have n coins and branch into paths using coins that are less than or equal to the current amount. Can you express this in terms of a recurrence relation? Also, try to determine the base condition to stop the recursion.

Hint 2

If the amount is 0, we return 0 coins. The recurrence relation can be expressed as min(1 + dfs(amount - coins[i])), where we return the minimum coins among all paths. This results in an O(n ^ t) solution, where n is the number of coins and t is the total amount. Can you think of a better approach? Perhaps consider the repeated work and find a way to avoid it.

Hint 3

We can use memoization to avoid the repeated work of calculating the result for each recursive call. A hash map or an array of size t can be used to cache the computed values for a specific amount. At each recursion step, we iterate over every coin and extend only the valid paths. If a result has already been computed, we return it from the cache instead of recalculating it.

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Prerequisites

Before attempting this problem, you should be comfortable with:

Recursion - Exploring all possible coin choices by reducing the problem to smaller subproblems
Dynamic Programming (Memoization) - Storing results of overlapping subproblems to optimize recursive solutions
Dynamic Programming (Tabulation) - Building the minimum coins array from smaller amounts to larger
Breadth-First Search (BFS) - Treating amounts as graph nodes to find the shortest path (minimum coins)

1. Recursion

Intuition

This is the pure recursive brute-force approach.

For a given amount, we try every coin:

Pick one coin
Solve the remaining subproblem amount - coin
Take the minimum coins needed among all choices

We explore all possible combinations, which leads to many repeated subproblems and exponential time — this solution is correct but inefficient.

If no combination reaches exactly 0, we treat it as invalid using a very large number.

Algorithm

Define a recursive function dfs(amount):
- If amount == 0, return 0 (no coins needed)
Initialize res as a very large value
For each coin:
- If amount - coin >= 0
  - Recursively compute 1 + dfs(amount - coin)
  - Update res with the minimum
Return res
If the final result is still very large, return -1, else return the result

class Solution:
    def coinChange(self, coins: List[int], amount: int) -> int:

        def dfs(amount):
            if amount == 0:
                return 0

            res = 1e9
            for coin in coins:
                if amount - coin >= 0:
                    res = min(res, 1 + dfs(amount - coin))
            return res

        minCoins = dfs(amount)
        return -1 if minCoins >= 1e9 else minCoins

public class Solution {
    public int dfs(int[] coins, int amount) {
        if (amount == 0) return 0;

        int res = (int) 1e9;
        for (int coin : coins) {
            if (amount - coin >= 0) {
                res = Math.min(res,
                      1 + dfs(coins, amount - coin));
            }
        }
        return res;
    }

    public int coinChange(int[] coins, int amount) {
        int minCoins = dfs(coins, amount);
        return (minCoins >= 1e9) ? -1 : minCoins;
    }
}

class Solution {
public:
    int dfs(vector<int>& coins, int amount) {
        if (amount == 0) return 0;

        int res = 1e9;
        for (int coin : coins) {
            if (amount - coin >= 0) {
                res = min(res,
                      1 + dfs(coins, amount - coin));
            }
        }
        return res;
    }

    int coinChange(vector<int>& coins, int amount) {
        int minCoins = dfs(coins, amount);
        return (minCoins >= 1e9) ? -1 : minCoins;
    }
};

class Solution {
    /**
     * @param {number[]} coins
     * @param {number} amount
     * @return {number}
     */
    coinChange(coins, amount) {
        const dfs = (amount) => {
            if (amount === 0) return 0;

            let res = Infinity;
            for (let coin of coins) {
                if (amount - coin >= 0) {
                    res = Math.min(res, 1 + dfs(amount - coin));
                }
            }
            return res;
        };

        const minCoins = dfs(amount);
        return minCoins === Infinity ? -1 : minCoins;
    }
}

public class Solution {
    public int Dfs(int[] coins, int amount) {
        if (amount == 0) return 0;

        int res = (int)1e9;
        foreach (var coin in coins) {
            if (amount - coin >= 0) {
                res = Math.Min(res,
                      1 + Dfs(coins, amount - coin));
            }
        }
        return res;
    }

    public int CoinChange(int[] coins, int amount) {
        int minCoins = Dfs(coins, amount);
        return (minCoins >= 1e9) ? -1 : minCoins;
    }
}

func coinChange(coins []int, amount int) int {
    var dfs func(int) int
    dfs = func(amt int) int {
        if amt == 0 {
            return 0
        }

        res := math.MaxInt32
        for _, coin := range coins {
            if amt - coin >= 0 {
                res = min(res, 1 + dfs(amt - coin))
            }
        }

        return res
    }

    minCoins := dfs(amount)
    if minCoins >= math.MaxInt32 {
        return -1
    }
    return minCoins
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

class Solution {
    fun coinChange(coins: IntArray, amount: Int): Int {
        val INF = 1000000000
        fun dfs(amt: Int): Int {
            if (amt == 0) return 0

            var res = INF
            for (coin in coins) {
                if (amt - coin >= 0) {
                    res = minOf(res, 1 + dfs(amt - coin))
                }
            }
            return res
        }

        val minCoins = dfs(amount)
        return if (minCoins >= INF) -1 else minCoins
    }
}

class Solution {
    func coinChange(_ coins: [Int], _ amount: Int) -> Int {
        func dfs(_ amount: Int) -> Int {
            if amount == 0 {
                return 0
            }

            var res = Int(1e9)
            for coin in coins {
                if amount - coin >= 0 {
                    res = min(res, 1 + dfs(amount - coin))
                }
            }
            return res
        }

        let minCoins = dfs(amount)
        return minCoins >= Int(1e9) ? -1 : minCoins
    }
}

impl Solution {
    pub fn coin_change(coins: Vec<i32>, amount: i32) -> i32 {
        fn dfs(coins: &[i32], amount: i32) -> i32 {
            if amount == 0 { return 0; }

            let mut res = 1_000_000_000;
            for &coin in coins {
                if amount - coin >= 0 {
                    res = res.min(1 + dfs(coins, amount - coin));
                }
            }
            res
        }

        let min_coins = dfs(&coins, amount);
        if min_coins >= 1_000_000_000 { -1 } else { min_coins }
    }
}

Time & Space Complexity

Time complexity: $O(n ^ t)$
Space complexity: $O(t)$

Where $n$ is the length of the array $coins$ and $t$ is the given $amount$ .

2. Dynamic Programming (Top-Down)

Intuition

This is the optimized version of the brute-force recursion using memoization.

The key observation is that the same amount gets solved multiple times in recursion.
So instead of recomputing it, we store the result the first time and reuse it.

Each amount represents a subproblem:

Minimum coins needed to make this amount

Algorithm

Use a hashmap memo to store results for already computed amounts
Define dfs(amount):
- If amount == 0, return 0
- If amount exists in memo, return it
Try every coin:
- If amount - coin >= 0
  - Compute 1 + dfs(amount - coin)
  - Track the minimum
Store the result in memo[amount]
Return the stored value
If the final answer is still very large, return -1

class Solution:
    def coinChange(self, coins: List[int], amount: int) -> int:
        memo = {}

        def dfs(amount):
            if amount == 0:
                return 0
            if amount in memo:
                return memo[amount]

            res = 1e9
            for coin in coins:
                if amount - coin >= 0:
                    res = min(res, 1 + dfs(amount - coin))

            memo[amount] = res
            return res

        minCoins = dfs(amount)
        return -1 if minCoins >= 1e9 else minCoins

public class Solution {
    HashMap<Integer, Integer> memo = new HashMap<>();

    public int dfs(int amount, int[] coins) {
        if (amount == 0) return 0;
        if (memo.containsKey(amount))
            return memo.get(amount);

        int res = Integer.MAX_VALUE;
        for (int coin : coins) {
            if (amount - coin >= 0) {
                int result = dfs(amount - coin, coins);
                if (result != Integer.MAX_VALUE) {
                    res = Math.min(res, 1 + result);
                }
            }
        }

        memo.put(amount, res);
        return res;
    }

    public int coinChange(int[] coins, int amount) {
        int minCoins = dfs(amount, coins);
        return minCoins == Integer.MAX_VALUE ? -1 : minCoins;
    }
}

class Solution {
public:
    unordered_map<int, int> memo;
    int dfs(int amount, vector<int>& coins) {
        if (amount == 0) return 0;
        if (memo.find(amount) != memo.end())
            return memo[amount];

        int res = INT_MAX;
        for (int coin : coins) {
            if (amount - coin >= 0) {
                int result = dfs(amount - coin, coins);
                if (result != INT_MAX) {
                    res = min(res, 1 + result);
                }
            }
        }

        memo[amount] = res;
        return res;
    }

    int coinChange(vector<int>& coins, int amount) {
        int minCoins = dfs(amount, coins);
        return minCoins == INT_MAX ? -1 : minCoins;
    }
};

class Solution {
    /**
     * @param {number[]} coins
     * @param {number} amount
     * @return {number}
     */
    coinChange(coins, amount) {
        let memo = {};

        const dfs = (amount) => {
            if (amount === 0) return 0;
            if (memo[amount] !== undefined) return memo[amount];

            let res = Infinity;
            for (let coin of coins) {
                if (amount - coin >= 0) {
                    res = Math.min(res, 1 + dfs(amount - coin));
                }
            }

            memo[amount] = res;
            return res;
        };

        const minCoins = dfs(amount);
        return minCoins === Infinity ? -1 : minCoins;
    }
}

public class Solution {
    private Dictionary<int, int> memo = new Dictionary<int, int>();

    private int Dfs(int amount, int[] coins) {
        if (amount == 0) return 0;
        if (memo.ContainsKey(amount))
            return memo[amount];

        int res = int.MaxValue;
        foreach (int coin in coins) {
            if (amount - coin >= 0) {
                int result = Dfs(amount - coin, coins);
                if (result != int.MaxValue) {
                    res = Math.Min(res, 1 + result);
                }
            }
        }

        memo[amount] = res;
        return res;
    }

    public int CoinChange(int[] coins, int amount) {
        int minCoins = Dfs(amount, coins);
        return minCoins == int.MaxValue ? -1 : minCoins;
    }
}

func coinChange(coins []int, amount int) int {
    var memo = make(map[int]int)
    memo[0] = 0

    var dfs func(int) int
    dfs = func(amt int) int {
        if val, ok := memo[amt]; ok {
            return val
        }

        res := math.MaxInt32
        for _, coin := range coins {
            if amt - coin >= 0 {
                res = min(res, 1 + dfs(amt - coin))
            }
        }
        memo[amt] = res
        return res
    }

    minCoins := dfs(amount)
    if minCoins >= math.MaxInt32 {
        return -1
    }
    return minCoins
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

class Solution {
    fun coinChange(coins: IntArray, amount: Int): Int {
        val INF = 1000000000
        var memo = hashMapOf(0 to 0)

        fun dfs(amt: Int): Int {
            if (amt in memo) {
                return memo[amt]!!
            }

            var res = INF
            for (coin in coins) {
                if (amt - coin >= 0) {
                    res = minOf(res, 1 + dfs(amt - coin))
                }
            }
            memo[amt] = res
            return res
        }

        val minCoins = dfs(amount)
        return if (minCoins >= INF) -1 else minCoins
    }
}

class Solution {
    func coinChange(_ coins: [Int], _ amount: Int) -> Int {
        var memo = [Int: Int]()

        func dfs(_ amount: Int) -> Int {
            if amount == 0 {
                return 0
            }
            if let cached = memo[amount] {
                return cached
            }

            var res = Int(1e9)
            for coin in coins {
                if amount - coin >= 0 {
                    res = min(res, 1 + dfs(amount - coin))
                }
            }

            memo[amount] = res
            return res
        }

        let minCoins = dfs(amount)
        return minCoins >= Int(1e9) ? -1 : minCoins
    }
}

Time & Space Complexity

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

Where $n$ is the length of the array $coins$ and $t$ is the given $amount$ .

3. Dynamic Programming (Bottom-Up)

Intuition

This is the bottom-up DP version of Coin Change.

Instead of asking “how many coins to make this amount?” recursively, we build answers from smaller amounts to larger ones.

Key idea:

If we know the minimum coins to make a - coin,
then we can make a using 1 extra coin.

So each amount depends on previously solved smaller amounts.

Algorithm

Create a DP array dp where dp[a] = minimum coins needed to make amount a
Initialize:
- dp[0] = 0 (0 coins to make amount 0)
- All other values as a large number (amount + 1)
For every amount a from 1 to amount:
- For each coin c:
  - If a - c >= 0
    - Update dp[a] = min(dp[a], 1 + dp[a - c])
If dp[amount] is still large, return -1
Otherwise return dp[amount]

class Solution:
    def coinChange(self, coins: List[int], amount: int) -> int:
        dp = [amount + 1] * (amount + 1)
        dp[0] = 0

        for a in range(1, amount + 1):
            for c in coins:
                if a - c >= 0:
                    dp[a] = min(dp[a], 1 + dp[a - c])
        return dp[amount] if dp[amount] != amount + 1 else -1

public class Solution {
    public int coinChange(int[] coins, int amount) {
        int[] dp = new int[amount + 1];
        Arrays.fill(dp, amount + 1);
        dp[0] = 0;
        for (int i = 1; i <= amount; i++) {
            for (int j = 0; j < coins.length; j++) {
                if (coins[j] <= i) {
                    dp[i] = Math.min(dp[i], dp[i - coins[j]] + 1);
                }
            }
        }
        return dp[amount] > amount ? -1 : dp[amount];
    }
}

class Solution {
public:
    int coinChange(vector<int>& coins, int amount) {
        vector<int> dp(amount + 1, amount + 1);
        dp[0] = 0;
        for (int i = 1; i <= amount; i++) {
            for (int j = 0; j < coins.size(); j++) {
                if (coins[j] <= i) {
                    dp[i] = min(dp[i], dp[i - coins[j]] + 1);
                }
            }
        }
        return dp[amount] > amount ? -1 : dp[amount];
    }
};

class Solution {
    /**
     * @param {number[]} coins
     * @param {number} amount
     * @return {number}
     */
    coinChange(coins, amount) {
        const dp = new Array(amount + 1).fill(amount + 1);
        dp[0] = 0;
        for (let i = 1; i <= amount; i++) {
            for (let j = 0; j < coins.length; j++) {
                if (coins[j] <= i) {
                    dp[i] = Math.min(dp[i], 1 + dp[i - coins[j]]);
                }
            }
        }
        return dp[amount] > amount ? -1 : dp[amount];
    }
}

public class Solution {
    public int CoinChange(int[] coins, int amount) {
        int[] dp = new int[amount + 1];
        Array.Fill(dp, amount + 1);
        dp[0] = 0;
        for (int i = 1; i <= amount; i++) {
            foreach (int coin in coins) {
                if (coin <= i) {
                    dp[i] = Math.Min(dp[i], dp[i - coin] + 1);
                }
            }
        }
        return dp[amount] > amount ? -1 : dp[amount];
    }
}

func coinChange(coins []int, amount int) int {
    dp := make([]int, amount+1)
    for i := range dp {
        dp[i] = amount + 1
    }
    dp[0] = 0
    for a := 1; a <= amount; a++ {
        for _, c := range coins {
            if a-c >= 0 {
                dp[a] = min(dp[a], 1+dp[a-c])
            }
        }
    }
    if dp[amount] > amount {
        return -1
    }
    return dp[amount]
}

func min(a, b int) int {
    if a < b {
        return a
    }
    return b
}

class Solution {
    fun coinChange(coins: IntArray, amount: Int): Int {
        val dp = IntArray(amount + 1) { amount + 1 }
        dp[0] = 0
        for (a in 1..amount) {
            for (c in coins) {
                if (a - c >= 0) {
                    dp[a] = minOf(dp[a], 1 + dp[a - c])
                }
            }
        }
        return if (dp[amount] > amount) -1 else dp[amount]
    }
}

class Solution {
    func coinChange(_ coins: [Int], _ amount: Int) -> Int {
        if amount == 0 {
            return 0
        }

        var dp = [Int](repeating: amount + 1, count: amount + 1)
        dp[0] = 0

        for a in 1...amount {
            for coin in coins {
                if a - coin >= 0 {
                    dp[a] = min(dp[a], 1 + dp[a - coin])
                }
            }
        }

        return dp[amount] == amount + 1 ? -1 : dp[amount]
    }
}

impl Solution {
    pub fn coin_change(coins: Vec<i32>, amount: i32) -> i32 {
        let amount = amount as usize;
        let mut dp = vec![amount as i32 + 1; amount + 1];
        dp[0] = 0;

        for a in 1..=amount {
            for &c in &coins {
                if c as usize <= a {
                    dp[a] = dp[a].min(dp[a - c as usize] + 1);
                }
            }
        }

        if dp[amount] > amount as i32 { -1 } else { dp[amount] }
    }
}

Time & Space Complexity

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

Where $n$ is the length of the array $coins$ and $t$ is the given $amount$ .

4. Breadth First Search

Intuition

Think of each amount as a node in a graph.

From a current amount x, you can go to x + coin for every coin.
Each edge represents using one coin.
We want the minimum number of coins, which means the shortest path from 0 to amount.

This makes the problem a shortest path in an unweighted graph, so Breadth First Search (BFS) is a natural fit.

BFS explores level by level:

Level 1 → amounts reachable using 1 coin
Level 2 → amounts reachable using 2 coins
First time we reach amount, we've used the minimum coins.

Algorithm

If amount == 0, return 0
Initialize a queue with 0 (starting amount)
Use a seen array to avoid revisiting amounts
Set steps = 0
While the queue is not empty:
- Increment steps (represents number of coins used)
- For each element in the current level:
  - Try adding every coin
  - If current + coin == amount, return steps
  - If within bounds and unseen, mark seen and push into queue
If BFS finishes without reaching amount, return -1

class Solution:
    def coinChange(self, coins: List[int], amount: int) -> int:
        if amount == 0:
            return 0

        q = deque([0])
        seen = [False] * (amount + 1)
        seen[0] = True
        res = 0

        while q:
            res += 1
            for _ in range(len(q)):
                cur = q.popleft()
                for coin in coins:
                    nxt = cur + coin
                    if nxt == amount:
                        return res
                    if nxt > amount or seen[nxt]:
                        continue
                    seen[nxt] = True
                    q.append(nxt)

        return -1

public class Solution {
    public int coinChange(int[] coins, int amount) {
        if (amount == 0) return 0;

        Queue<Integer> q = new LinkedList<>();
        q.add(0);
        boolean[] seen = new boolean[amount + 1];
        seen[0] = true;
        int res = 0;

        while (!q.isEmpty()) {
            res++;
            int size = q.size();
            for (int i = 0; i < size; i++) {
                int cur = q.poll();
                for (int coin : coins) {
                    int nxt = cur + coin;
                    if (nxt == amount) return res;
                    if (nxt > amount || seen[nxt]) continue;
                    seen[nxt] = true;
                    q.add(nxt);
                }
            }
        }

        return -1;
    }
}

class Solution {
public:
    int coinChange(vector<int>& coins, int amount) {
        if (amount == 0) return 0;

        queue<int> q;
        q.push(0);
        vector<bool> seen(amount + 1, false);
        seen[0] = true;
        int res = 0;

        while (!q.empty()) {
            res++;
            int size = q.size();
            for (int i = 0; i < size; i++) {
                int cur = q.front();
                q.pop();
                for (int coin : coins) {
                    int nxt = cur + coin;
                    if (nxt == amount) return res;
                    if (nxt > amount || seen[nxt]) continue;
                    seen[nxt] = true;
                    q.push(nxt);
                }
            }
        }

        return -1;
    }
};

class Solution {
    /**
     * @param {number[]} coins
     * @param {number} amount
     * @return {number}
     */
    coinChange(coins, amount) {
        if (amount === 0) return 0;

        const q = new Queue([0]);
        const seen = new Array(amount + 1).fill(false);
        seen[0] = true;
        let res = 0;

        while (!q.isEmpty()) {
            res++;
            const size = q.size();
            for (let i = 0; i < size; i++) {
                const cur = q.pop();
                for (const coin of coins) {
                    const nxt = cur + coin;
                    if (nxt === amount) return res;
                    if (nxt > amount || seen[nxt]) continue;
                    seen[nxt] = true;
                    q.push(nxt);
                }
            }
        }

        return -1;
    }
}

public class Solution {
    public int CoinChange(int[] coins, int amount) {
        if (amount == 0) return 0;

        Queue<int> q = new Queue<int>();
        q.Enqueue(0);
        bool[] seen = new bool[amount + 1];
        seen[0] = true;
        int res = 0;

        while (q.Count > 0) {
            res++;
            int size = q.Count;
            for (int i = 0; i < size; i++) {
                int cur = q.Dequeue();
                foreach (int coin in coins) {
                    int nxt = cur + coin;
                    if (nxt == amount) return res;
                    if (nxt > amount || seen[nxt]) continue;
                    seen[nxt] = true;
                    q.Enqueue(nxt);
                }
            }
        }

        return -1;
    }
}

func coinChange(coins []int, amount int) int {
    if amount == 0 {
        return 0
    }
    queue := []int{0}
    seen := make([]bool, amount+1)
    seen[0] = true
    res := 0
    for len(queue) > 0 {
        res++
        size := len(queue)
        for i := 0; i < size; i++ {
            cur := queue[0]
            queue = queue[1:]
            for _, coin := range coins {
                next := cur + coin
                if next == amount {
                    return res
                }
                if next > amount || seen[next] {
                    continue
                }
                seen[next] = true
                queue = append(queue, next)
            }
        }
    }
    return -1
}

class Solution {
    fun coinChange(coins: IntArray, amount: Int): Int {
        if (amount == 0) return 0
        val queue = ArrayDeque<Int>().apply { add(0) }
        val seen = BooleanArray(amount + 1) { false }
        seen[0] = true
        var res = 0
        while (queue.isNotEmpty()) {
            res++
            repeat(queue.size) {
                val cur = queue.removeFirst()
                for (coin in coins) {
                    val next = cur + coin
                    if (next == amount) return res
                    if (next > amount || seen[next]) continue
                    seen[next] = true
                    queue.addLast(next)
                }
            }
        }
        return -1
    }
}

class Solution {
    func coinChange(_ coins: [Int], _ amount: Int) -> Int {
        if amount == 0 {
            return 0
        }

        var q = Deque([0])
        var seen = Array(repeating: false, count: amount + 1)
        seen[0] = true
        var res = 0

        while !q.isEmpty {
            res += 1
            for _ in 0..<q.count {
                let cur = q.popFirst()!
                for coin in coins {
                    let nxt = cur + coin
                    if nxt == amount {
                        return res
                    }
                    if nxt > amount || seen[nxt] {
                        continue
                    }
                    seen[nxt] = true
                    q.append(nxt)
                }
            }
        }

        return -1
    }
}

impl Solution {
    pub fn coin_change(coins: Vec<i32>, amount: i32) -> i32 {
        if amount == 0 { return 0; }

        let amount = amount as usize;
        let mut q = VecDeque::new();
        q.push_back(0usize);
        let mut seen = vec![false; amount + 1];
        seen[0] = true;
        let mut res = 0;

        while !q.is_empty() {
            res += 1;
            let size = q.len();
            for _ in 0..size {
                let cur = q.pop_front().unwrap();
                for &coin in &coins {
                    let nxt = cur + coin as usize;
                    if nxt == amount { return res; }
                    if nxt > amount || seen[nxt] { continue; }
                    seen[nxt] = true;
                    q.push_back(nxt);
                }
            }
        }

        -1
    }
}

Time & Space Complexity

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

Where $n$ is the length of the array $coins$ and $t$ is the given $amount$ .

Common Pitfalls

Using Greedy Instead of DP

A common mistake is assuming a greedy approach (always picking the largest coin) works. For coins = [1, 3, 4] and amount = 6, greedy picks [4, 1, 1] (3 coins), but the optimal is [3, 3] (2 coins). The problem requires exploring all combinations.

Incorrect Handling of Impossible Cases

Forgetting to return -1 when the amount cannot be formed. Using 0 or leaving the result as infinity/large value causes wrong answers.

# Wrong: returns large number instead of -1
return dp[amount]
# Correct: check if amount is reachable
return dp[amount] if dp[amount] != amount + 1 else -1

Integer Overflow When Adding 1 to Infinity

When using INT_MAX or Integer.MAX_VALUE as the "impossible" sentinel, adding 1 causes overflow. Use amount + 1 as the sentinel instead, since you can never need more than amount coins (when using coin value 1).

// Wrong: 1 + INT_MAX overflows
if (result != INT_MAX) res = min(res, 1 + result);
// Correct: use amount + 1 as sentinel
int[] dp = new int[amount + 1];
Arrays.fill(dp, amount + 1);