问题描述：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".
Examples：

Input: J = "aA", S = "aAAbbbb"
Output: 3

思路：利用对象的属性来判断，把J中的每一个字符定义为某个对象的属性，然后遍历S判断S中的每一个字符是在这个对象的属性中

/**
 * @param {string} J
 * @param {string} S
 * @return {number}
 */
var numJewelsInStones = function(J, S) {
    var ret = {};
    for(j in J){
        ret[J[j]] = 1;
    }
    var num = 0;
    for(i in S){
        if(S[i] in ret){
            num++;
        }
    }
    return num;
};

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风靡一时的美国招兵海报

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