用字符数组ch来记录magazine每个字母出现频率，用ransomNote的字母减去字符数组ch对应的字符出现频率，如果该字符对应的频率小于0，则不够，无法组成ransomNote！

class Solution {
public:bool canConstruct(string ransomNote, string magazine) {//unordered_map<char,int> umap;int ch[27];if(ransomNote.size()>magazine.size()){return false;}for(const auto& c:magazine){++ch[c-'a'];}for(const auto& c:ransomNote){--ch[c-'a'];if(ch[c-'a']<0){return false;}}return true;}
};