Longest Palindromic Substring (LeetCode)

Problem: Longest Palindromic Substring

Given a string S, find the longest palindromic substring in S.

class Solution {
public:
    string longestPalindrome(string s) {
        int mx=0, start=-1;
        for(int i=0;i<s.size();i++)
        {
            int mid = expandCenter(s, i, i);
            if(mx < mid)
            {
                mx=mid;
                start = i-mx/2;
            }
            int left  = expandCenter(s, i, i+1);
            if(mx < left)
            {
                mx=left;
                start = i-mx/2+1;
            }
        }
        return s.substr(start, mx);
    }
    
    int expandCenter(string s, int left, int right)
    {
        while(left>=0 && right<s.size() && s[left]==s[right])
        {
            left--, right++;
        }
        return right-left-1;
    }
    
};