Problem description:

You’re given strings J representing the types of stones that are jewels, and S representing the stones you have. Each character in S is 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.

Solution:

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

time complexity: $O(n)$
space complexity: $O(n)$
reference: