Jewels and Stones

Problem Statement:

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

Solution:
class Solution:
    def numJewelsInStones(self, J: str, S: str) -> int:
        cnt=0
        for i in S:
            if i in J:
                cnt+=1
        return cnt