1160. Find Words That Can Be Formed by Characters - Explanation

Prerequisites

Before attempting this problem, you should be comfortable with:

Hash Maps/Dictionaries - Counting character frequencies and performing lookups
String Iteration - Traversing characters in strings to build frequency counts
Arrays as Fixed-Size Maps - Using arrays of size 26 for lowercase letter counting as an optimization

1. Hash Map (Two Pass)

Intuition

A word can be formed from chars if every character in the word appears in chars with at least the same frequency. We first count character frequencies in chars, then for each word, count its character frequencies and verify that chars has enough of each character.

Algorithm

Build a frequency map count for all characters in chars.
For each word:
- Build a frequency map cur_word for the word.
- Check if every character in cur_word has a count less than or equal to its count in count.
- If valid, add the word's length to res.
Return the total length.

class Solution:
    def countCharacters(self, words: List[str], chars: str) -> int:
        count = Counter(chars)
        res = 0

        for w in words:
            cur_word = Counter(w)
            good = True
            for c in cur_word:
                if cur_word[c] > count[c]:
                    good = False
                    break
            if good:
                res += len(w)
        return res

public class Solution {
    public int countCharacters(String[] words, String chars) {
        Map<Character, Integer> count = new HashMap<>();
        for (char c : chars.toCharArray()) {
            count.put(c, count.getOrDefault(c, 0) + 1);
        }
        int res = 0;
        for (String w : words) {
            Map<Character, Integer> curWord = new HashMap<>();
            for (char c : w.toCharArray()) {
                curWord.put(c, curWord.getOrDefault(c, 0) + 1);
            }
            boolean good = true;
            for (char c : curWord.keySet()) {
                if (curWord.get(c) > count.getOrDefault(c, 0)) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.length();
            }
        }
        return res;
    }
}

class Solution {
public:
    int countCharacters(vector<string>& words, string chars) {
        unordered_map<char, int> count;
        for (char c : chars) {
            count[c]++;
        }
        int res = 0;
        for (const string& w : words) {
            unordered_map<char, int> curWord;
            for (char c : w) {
                curWord[c]++;
            }
            bool good = true;
            for (const auto& p : curWord) {
                if (p.second > count[p.first]) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.size();
            }
        }
        return res;
    }
};

class Solution {
    /**
     * @param {string[]} words
     * @param {string} chars
     * @return {number}
     */
    countCharacters(words, chars) {
        const count = {};
        for (const c of chars) {
            count[c] = (count[c] || 0) + 1;
        }
        let res = 0;
        for (const w of words) {
            const curWord = {};
            for (const c of w) {
                curWord[c] = (curWord[c] || 0) + 1;
            }
            let good = true;
            for (const c in curWord) {
                if (curWord[c] > (count[c] || 0)) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.length;
            }
        }
        return res;
    }
}

public class Solution {
    public int CountCharacters(string[] words, string chars) {
        Dictionary<char, int> count = new Dictionary<char, int>();
        foreach (char c in chars) {
            if (!count.ContainsKey(c)) count[c] = 0;
            count[c]++;
        }
        int res = 0;
        foreach (string w in words) {
            Dictionary<char, int> curWord = new Dictionary<char, int>();
            foreach (char c in w) {
                if (!curWord.ContainsKey(c)) curWord[c] = 0;
                curWord[c]++;
            }
            bool good = true;
            foreach (var kvp in curWord) {
                int available = count.ContainsKey(kvp.Key) ? count[kvp.Key] : 0;
                if (kvp.Value > available) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.Length;
            }
        }
        return res;
    }
}

func countCharacters(words []string, chars string) int {
    count := make(map[rune]int)
    for _, c := range chars {
        count[c]++
    }
    res := 0
    for _, w := range words {
        curWord := make(map[rune]int)
        for _, c := range w {
            curWord[c]++
        }
        good := true
        for c, cnt := range curWord {
            if cnt > count[c] {
                good = false
                break
            }
        }
        if good {
            res += len(w)
        }
    }
    return res
}

class Solution {
    fun countCharacters(words: Array<String>, chars: String): Int {
        val count = mutableMapOf<Char, Int>()
        for (c in chars) {
            count[c] = count.getOrDefault(c, 0) + 1
        }
        var res = 0
        for (w in words) {
            val curWord = mutableMapOf<Char, Int>()
            for (c in w) {
                curWord[c] = curWord.getOrDefault(c, 0) + 1
            }
            var good = true
            for ((c, cnt) in curWord) {
                if (cnt > count.getOrDefault(c, 0)) {
                    good = false
                    break
                }
            }
            if (good) {
                res += w.length
            }
        }
        return res
    }
}

class Solution {
    func countCharacters(_ words: [String], _ chars: String) -> Int {
        var count = [Character: Int]()
        for c in chars {
            count[c, default: 0] += 1
        }
        var res = 0
        for w in words {
            var curWord = [Character: Int]()
            for c in w {
                curWord[c, default: 0] += 1
            }
            var good = true
            for (c, cnt) in curWord {
                if cnt > (count[c] ?? 0) {
                    good = false
                    break
                }
            }
            if good {
                res += w.count
            }
        }
        return res
    }
}

Time & Space Complexity

Time complexity: $O(n + (m * k))$
Space complexity: $O(1)$ since we have at most $26$ different characters.

Where $n$ is the length of $chars$ , $m$ is the number of words and $k$ is the average length of each word.

2. Hash Map (One Pass)

Intuition

Instead of building the complete frequency map for each word first, we can check character availability on the fly. As we iterate through each character of a word, we increment its count in a temporary map and immediately check if it exceeds the available count in chars. This allows early termination if a word is invalid.

Algorithm

Build a frequency map count for all characters in chars.
For each word:
- Initialize an empty frequency map cur_word.
- For each character in the word:
  - Increment its count in cur_word.
  - If it exceeds the count in count, mark the word as invalid and break.
- If the word is valid, add its length to res.
Return the total length.

class Solution:
    def countCharacters(self, words: List[str], chars: str) -> int:
        count = Counter(chars)
        res = 0

        for w in words:
            cur_word = defaultdict(int)
            good = True
            for c in w:
                cur_word[c] += 1
                if cur_word[c] > count[c]:
                    good = False
                    break
            if good:
                res += len(w)
        return res

public class Solution {
    public int countCharacters(String[] words, String chars) {
        Map<Character, Integer> count = new HashMap<>();
        for (char c : chars.toCharArray()) {
            count.put(c, count.getOrDefault(c, 0) + 1);
        }
        int res = 0;
        for (String w : words) {
            Map<Character, Integer> curWord = new HashMap<>();
            boolean good = true;
            for (char c : w.toCharArray()) {
                curWord.put(c, curWord.getOrDefault(c, 0) + 1);
                if (curWord.get(c) > count.getOrDefault(c, 0)) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.length();
            }
        }
        return res;
    }
}

class Solution {
public:
    int countCharacters(vector<string>& words, string chars) {
        unordered_map<char, int> count;
        for (char c : chars) {
            count[c]++;
        }
        int res = 0;
        for (const string& w : words) {
            unordered_map<char, int> curWord;
            bool good = true;
            for (char c : w) {
                curWord[c]++;
                if (curWord[c] > count[c]) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.size();
            }
        }
        return res;
    }
};

class Solution {
    /**
     * @param {string[]} words
     * @param {string} chars
     * @return {number}
     */
    countCharacters(words, chars) {
        const count = {};
        for (const c of chars) {
            count[c] = (count[c] || 0) + 1;
        }
        let res = 0;
        for (const w of words) {
            const curWord = {};
            let good = true;
            for (const c of w) {
                curWord[c] = (curWord[c] || 0) + 1;
                if (curWord[c] > (count[c] || 0)) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.length;
            }
        }
        return res;
    }
}

public class Solution {
    public int CountCharacters(string[] words, string chars) {
        Dictionary<char, int> count = new Dictionary<char, int>();
        foreach (char c in chars) {
            if (!count.ContainsKey(c)) count[c] = 0;
            count[c]++;
        }
        int res = 0;
        foreach (string w in words) {
            Dictionary<char, int> curWord = new Dictionary<char, int>();
            bool good = true;
            foreach (char c in w) {
                if (!curWord.ContainsKey(c)) curWord[c] = 0;
                curWord[c]++;
                int available = count.ContainsKey(c) ? count[c] : 0;
                if (curWord[c] > available) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.Length;
            }
        }
        return res;
    }
}

func countCharacters(words []string, chars string) int {
    count := make(map[rune]int)
    for _, c := range chars {
        count[c]++
    }
    res := 0
    for _, w := range words {
        curWord := make(map[rune]int)
        good := true
        for _, c := range w {
            curWord[c]++
            if curWord[c] > count[c] {
                good = false
                break
            }
        }
        if good {
            res += len(w)
        }
    }
    return res
}

class Solution {
    fun countCharacters(words: Array<String>, chars: String): Int {
        val count = mutableMapOf<Char, Int>()
        for (c in chars) {
            count[c] = count.getOrDefault(c, 0) + 1
        }
        var res = 0
        for (w in words) {
            val curWord = mutableMapOf<Char, Int>()
            var good = true
            for (c in w) {
                curWord[c] = curWord.getOrDefault(c, 0) + 1
                if (curWord[c]!! > count.getOrDefault(c, 0)) {
                    good = false
                    break
                }
            }
            if (good) {
                res += w.length
            }
        }
        return res
    }
}

class Solution {
    func countCharacters(_ words: [String], _ chars: String) -> Int {
        var count = [Character: Int]()
        for c in chars {
            count[c, default: 0] += 1
        }
        var res = 0
        for w in words {
            var curWord = [Character: Int]()
            var good = true
            for c in w {
                curWord[c, default: 0] += 1
                if curWord[c]! > (count[c] ?? 0) {
                    good = false
                    break
                }
            }
            if good {
                res += w.count
            }
        }
        return res
    }
}

Time & Space Complexity

Time complexity: $O(n + (m * k))$
Space complexity: $O(1)$ since we have at most $26$ different characters.

Where $n$ is the length of $chars$ , $m$ is the number of words and $k$ is the average length of each word.

3. Hash Table

Intuition

Since we only have lowercase letters, we can use a fixed-size array of 26 elements instead of a hash map. This is more cache-friendly and avoids hash function overhead. We decrement counts as we use characters and reset the array for each new word using a stored original copy.

Algorithm

Create an array count of size 26 and populate it with frequencies from chars.
Store a copy of count as org for resetting.
For each word:
- For each character, decrement count[c - 'a'].
- If it goes negative, the word is invalid; break early.
- If valid, add the word's length to res.
- Reset count to org for the next word.
Return the total length.

class Solution:
    def countCharacters(self, words: List[str], chars: str) -> int:
        count = [0] * 26
        for c in chars:
            count[ord(c) - ord('a')] += 1

        org = count[:]
        res = 0

        for w in words:
            good = True
            for c in w:
                i = ord(c) - ord('a')
                count[i] -= 1
                if count[i] < 0:
                    good = False
                    break
            if good:
                res += len(w)

            for i in range(26):
                count[i] = org[i]
        return res

public class Solution {
    public int countCharacters(String[] words, String chars) {
        int[] count = new int[26];
        for (char c : chars.toCharArray()) {
            count[c - 'a']++;
        }

        int[] org = count.clone();
        int res = 0;

        for (String w : words) {
            boolean good = true;
            for (int i = 0; i < w.length(); i++) {
                int j = w.charAt(i) - 'a';
                count[j]--;
                if (count[j] < 0) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.length();
            }
            for (int i = 0; i < 26; i++) {
                count[i] = org[i];
            }
        }
        return res;
    }
}

class Solution {
public:
    int countCharacters(vector<string>& words, string chars) {
        vector<int> count(26, 0);
        for (char c : chars) {
            count[c - 'a']++;
        }

        vector<int> org = count;
        int res = 0;

        for (string& w : words) {
            bool good = true;
            for (char& c : w) {
                int i = c - 'a';
                count[i]--;
                if (count[i] < 0) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.length();
            }
            for (int i = 0; i < 26; i++) {
                count[i] = org[i];
            }
        }
        return res;
    }
};

class Solution {
    /**
     * @param {string[]} words
     * @param {string} chars
     * @return {number}
     */
    countCharacters(words, chars) {
        const count = new Array(26).fill(0);
        for (let c of chars) {
            count[c.charCodeAt(0) - 'a'.charCodeAt(0)]++;
        }

        const org = [...count];
        let res = 0;

        for (let w of words) {
            let good = true;
            for (let c of w) {
                const i = c.charCodeAt(0) - 'a'.charCodeAt(0);
                count[i]--;
                if (count[i] < 0) {
                    good = false;
                    break;
                }
            }

            if (good) {
                res += w.length;
            }
            for (let i = 0; i < 26; i++) {
                count[i] = org[i];
            }
        }
        return res;
    }
}

public class Solution {
    public int CountCharacters(string[] words, string chars) {
        int[] count = new int[26];
        foreach (char c in chars) {
            count[c - 'a']++;
        }

        int[] org = (int[])count.Clone();
        int res = 0;

        foreach (string w in words) {
            bool good = true;
            foreach (char c in w) {
                int i = c - 'a';
                count[i]--;
                if (count[i] < 0) {
                    good = false;
                    break;
                }
            }
            if (good) {
                res += w.Length;
            }
            for (int i = 0; i < 26; i++) {
                count[i] = org[i];
            }
        }
        return res;
    }
}

func countCharacters(words []string, chars string) int {
    count := make([]int, 26)
    for _, c := range chars {
        count[c-'a']++
    }

    org := make([]int, 26)
    copy(org, count)
    res := 0

    for _, w := range words {
        good := true
        for _, c := range w {
            i := c - 'a'
            count[i]--
            if count[i] < 0 {
                good = false
                break
            }
        }
        if good {
            res += len(w)
        }
        copy(count, org)
    }
    return res
}

class Solution {
    fun countCharacters(words: Array<String>, chars: String): Int {
        val count = IntArray(26)
        for (c in chars) {
            count[c - 'a']++
        }

        val org = count.copyOf()
        var res = 0

        for (w in words) {
            var good = true
            for (c in w) {
                val i = c - 'a'
                count[i]--
                if (count[i] < 0) {
                    good = false
                    break
                }
            }
            if (good) {
                res += w.length
            }
            for (i in 0 until 26) {
                count[i] = org[i]
            }
        }
        return res
    }
}

class Solution {
    func countCharacters(_ words: [String], _ chars: String) -> Int {
        var count = [Int](repeating: 0, count: 26)
        let aValue = Int(Character("a").asciiValue!)
        for c in chars {
            count[Int(c.asciiValue!) - aValue] += 1
        }

        let org = count
        var res = 0

        for w in words {
            var good = true
            for c in w {
                let i = Int(c.asciiValue!) - aValue
                count[i] -= 1
                if count[i] < 0 {
                    good = false
                    break
                }
            }
            if good {
                res += w.count
            }
            count = org
        }
        return res
    }
}

Time & Space Complexity

Time complexity: $O(n + (m * k))$
Space complexity: $O(1)$ since we have at most $26$ different characters.

Where $n$ is the length of $chars$ , $m$ is the number of words and $k$ is the average length of each word.

Common Pitfalls

Modifying the Original Character Count Map

When checking each word, you must use a fresh copy of the character counts or reset after each word. A common mistake is decrementing the original count map without restoring it, causing subsequent words to have fewer available characters than they should.

Checking Character Presence Without Frequency

It is not enough to check if a character exists in chars; you must verify that the frequency of each character in the word does not exceed its frequency in chars. For example, "aab" cannot be formed from "ab" even though both 'a' and 'b' are present.