leetcode——5——Longest Palindromic Substring

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

class Solution {public: string str_helper(string s, int left, int right)      {          int n = s.length();          while(left>=0 && right<=n-1 && s[left]==s[right]){              left--;              right++;          }          return s.substr(left+1, right-left-1);                }      string longestPalindrome(string s) {              if(s.length() <= 1)                  return s;          string longest = s.substr(0,1);          for (int i=0; i<s.length(); i++){                            string tmp = str_helper(s, i, i);              if (tmp.length() > longest.length())  {                  longest = tmp;              }              tmp = str_helper(s, i, i+1);              if (tmp.length() > longest.length()) {                  longest = tmp;              }          }                return longest;      } };