LeetCode 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) {
         map<char, int> visited;
         for(int i=0; i<magazine.size(); i++)
             visited[magazine[i]]++;
         for(int i=0; i<ransomNote.size(); i++){
             visited[ransomNote[i]]--;
             if(visited[ransomNote[i]]<0)
                 return false;
         }
        return true;
    }
};