C++

1class Solution {
2public:
3    int numJewelsInStones(string J, string S) {
4        int cnt = 0;
5        for(int i=0; S[i]; ++i) 
6        {
7            for(int j=0; j<J.size(); ++j) 
8            {
9                if(S[i] == J[j]) ++cnt;
10            }
11        }
12        
13        return cnt;
14    }
15};

JavaScript

1var numJewelsInStones = function(J, S) {
2    let cnt = 0;
3    S = S.split('');
4    S.forEach(c => {
5        (J.includes(c)) ? cnt += 1 : cnt;
6    })
7    
8    return cnt;
9};

Ruby

1def num_jewels_in_stones(j, s)
2    cnt = 0
3    s = s.split('')
4    for c in s do
5        if j.include?(c) then cnt += 1 end
6    end
7    cnt
8end