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

题目含义：判断给定字符串能否由另一字符串中字符组合生成

    public boolean canConstruct(String ransomNote, String magazine) {
        Map<Character,Integer> letterMap = new HashMap<>();
        for (Character c:magazine.toCharArray())
        {
            letterMap.put(c,letterMap.getOrDefault(c,0)+1);
        }
        
        for (Character c:ransomNote.toCharArray())
        {
            int count = letterMap.getOrDefault(c,0);
            if (count<=0) return false;
            letterMap.put(c,--count);
        }
        return true;        
    }