这是LeetCode的一个easy型题，easy型一般是用数据结构可以解决的问题，然而，长久不用脑子不写代码的话自然会生疏。so，please keep your brain running ！Make it higher，faster，stronger！

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

Note:

• S and J will consist of letters and have length at most 50.
• The characters in J are distinct.

解决方案：

class Solution {
public:
    int numJewelsInStones(string J, string S) {
        int count = 0;
        std::unordered_set<char> jewls;
        for (auto ch : J) {
            jewls.insert(ch);
        }
        std::unordered_set<char>::const_iterator got;
        for (auto ch : S){
            got = jewls.find(ch);
            if(got != jewls.end())
                count++;
        }
        return count;
    }
};