func numJewelsInStones(J string, S string) int {
    isJewel := make(map[byte]bool, len(J))
    count := 0
    for j := range J {
        isJewel[J[j]] = true
    }

    for s := range S {
        if isJewel[S[s]] {
            count ++
        }
    }
    return count
}