383. Ransom Note

Given an arbitrary ransom note string and another string containing letters from all the magazines, write a function that will return true if the ransom note can be constructed from the magazines ; otherwise, it will return false.

Each letter in the magazine string can only be used once in your ransom note.

Note: You may assume that both strings contain only lowercase letters.

canConstruct("a", "b") -> false
canConstruct("aa", "ab") -> false
canConstruct("aa", "aab") -> true
bool canConstruct(string ransomNote, string magazine) { // time: O(m + n); space: O(1)
    if (ransomNote.length() > magazine.length()) return false;
    vector<int> count(26, 0);
    for (char ch : magazine) ++count[ch - 'a'];
    for (char ch : ransomNote) {
        if (--count[ch - 'a'] < 0) return false;
    }
    return true;
}

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