Longest Palindromic Substring [LeetCode]

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

Summary: line 18, reduce unneccessary search

    bool isPalindrome(string &s, int start, int end){
        while(start < end){
            if(s[start] != s[end])
                return false;
            start ++;
            end --;
        }
        return true;
    }
    
    string longestPalindrome(string s) {
        int longest = 0;
        int start = 0;
        int end = 0;
        for(int i = 0; i < s.size(); i ++){
            //find s[i] from end of S
            for(int j = s.size() - 1; j >= i; j --){
                if(j - i + 1 <= longest)
                    break;
                if(isPalindrome(s, i, j)){
                    if(j - i + 1 > longest){
                        longest = j - i + 1;
                        start = i;
                        end = j;
                    }
                    break;
                }
            }
        }
        return s.substr(start, longest);
    }