求字符串有多少个回文子串

不好理解

class Solution {
public:
    int countSubstrings(string s) {
        int n = s.size(), sum = 0;
        for(int i = 0; i < n*2-1; i++)
        {
            int l = i / 2, r = i/2 + i%2;
            while(l>=0 && r < n && s[l] == s[r])
            {
                l--;
                r++;
                sum++;
            }
        }
        return sum;
    }
};