1. var countSubstrings = function (s) {
    2.     let count = 0;
    3.     for (let i = 0; i < s.length; i++) {
    4.         let s1 = '', s2 = '';
    6.         for (let j = i; j < s.length; j++) {
    8.             s1 += s[j], s2 = s[j] + s2;
    10.             if (s1 === s2) count++;
    11.         }
    12.     }
    14.     return count;
    15. };
    17. 链接：https://leetcode-cn.com/problems/a7VOhD/solution/jssan-jie-bao-li-shuang-zhi-zhen-zhong-x-qqq0/