771jewels and stones

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.

理用哈希法可以大幅加快查找速度。

class Solution {
    public int numJewelsInStones(String J, String S) {
        if(J.length()==0||S.length()==0||J==null||S==null){
            return 0;
        }
        int[]hash=new int[256];
        for(char c:J.toCharArray()){
            hash[c]++;
        }
        int res=0;
        for(char s:S.toCharArray()){
            if(hash[s]>0){
                res++;
            }
        }
        return res;
    }
}