1. 巧解
     for(let s of str){
     if(str.indexOf(s) === str.lastIndexOf(s)){
         return str.indexOf(s);
     }
 }
 return -1;
  2. 使用哈希表
    https://leetcode-cn.com/problems/di-yi-ge-zhi-chu-xian-yi-ci-de-zi-fu-lcof/solution/di-yi-ge-zhi-chu-xian-yi-ci-de-zi-fu-by-3zqv5/