771.md

Split the string J, representing the subset of all stones which are jewels, into a set where each element within that set represents a jewel. For each stone within the multiset S (represented as a string), determine whether that stone is indeed a jewel. Maintain a count of the number of stones within the multiset S which are jewels.

class Solution:
    def numJewelsInStones(self, J: str, S: str) -> int:
        no_jewels = 0
        
        jewels = set([jewel for jewel in J])
        
        for stone in S:
            if stone in jewels:
                no_jewels += 1
                
        return no_jewels

This can be rewritten to employ Python list comprehension:

class Solution:
    def numJewelsInStones(self, J: str, S: str) -> int:        
        jewels = set([jewel for jewel in J])
        return len([1 for stone in S if stone in jewels])