383. Ransom Note

Q:

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

A:

public class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
        Map<Character, Integer> map = new HashMap<>();
        for(char ch : magazine.toCharArray()){
            map.put(ch, map.containsKey(ch)?map.get(ch)+1:1 );
        }
        for(char ch : ransomNote.toCharArray()){
            if(!map.containsKey(ch)){
                return false;
            }
            map.put(ch, map.get(ch)-1);
            if(map.get(ch)==0)
                map.remove(ch);
        }
        return true;
    }
}

Errors :