771. Jewels and Stones

You're given strings J representing the types of stones that are jewels, and S representing the stones you have.  Each character in Sis a type of stone you have.  You want to know how many of the stones you have are also jewels.

The letters in J are guaranteed distinct, and all characters in J and S are letters. Letters are case sensitive, so "a" is considered a different type of stone from "A".

Example 1:

Input: J = "aA", S = "aAAbbbb"
Output: 3

Example 2:

Input: J = "z", S = "ZZ"
Output: 0


统计字符串J中的每一个字符在S中出现的总次数之和


C++(13ms)：

class Solution {
public:
    int numJewelsInStones(string J, string S) {
        int res = 0 ;
        for(char c1 : J){
            for(char c2 : S){
                if (c1 == c2)
                    res++ ;
            }
        }
        return res ;
    }
};

C++(14ms):

class Solution {
public:
    int numJewelsInStones(string J, string S) {
        int res = 0 ;
        unordered_set<char> setJ(J.begin() , J.end()) ;
        for(char c : S){
            if (setJ.count(c))
                res++ ;
        }
        return res ;
    }
};