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.

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canConstruct("a", "b") -> false
canConstruct("aa", "ab") -> false
canConstruct("aa", "aab") -> true

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class Solution {
public:
    bool canConstruct(string ransomNote, string magazine) {
        bool result = true;
        unordered_map<char, int> m;
        for (int i = 0, len = magazine.length(); i < len; i++) {
            m[magazine[i]]++;
        }
        for (int j = 0, len = ransomNote.length(); j < len; j++) {
            if (m[ransomNote[j]]-- == 0) {
                result = false;
                break;
            }
        }
        return result;
    }
};