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

class Solution {
public:
    bool canConstruct(string ransomNote, string magazine) {
        unordered_map<char,int> hashMap;
        for(char ch : magazine){
            ++hashMap[ch];
        }
        
        for(char ch :ransomNote){
            if(--hashMap[ch]<0)
                return false;
        }
        
        return true;
    }
};