leetcode 5. Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

#include<cmath>
class Solution {
public:
    string longestPalindrome(string s) {
        int  start=0, end=0, i ;
        for(i=0; i<s.size(); i++){
            int len1=sub_string_length(s, i, i);
            int len2=sub_string_length(s, i, i+1);
            int len = len1>len2 ? len1 : len2;
            if(len>end-start+1){
                start=i-(len-1)/2;
                end=i+len/2;
            }
        }
        return s.substr(start, end-start+1);
    }
    
    int sub_string_length(string s, int left, int right){
        int l=left, r=right;
        while(l>=0 && r<s.size() && s[l]==s[r]){
            l--;
            r++;
        }
        return r-l-1;
    }
};