771. Jewels and Stones

771. Jewels 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 stones you have. You want to know how many of 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".

public int numJewelsInStones(String J,String S) {
      return S.replaceAll("[^"+J+"]","").length();
}

小结，用到了字符串的替换，替换的内容是正则表达式
此解法时间复杂度和空间复杂度都不是最低的。
[^a-z] //除了小写字母以外的所有字符,附https://www.runoob.com/regexp/regexp-rule.html