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:

Input: "babad"

Output: "bab"

Note: "aba" is also a valid answer.

Example:

Input: "cbbd"

Output: "bb"

class Solution {
public:
    string longestPalindrome(string s){
      int len=s.size();
      string s1(1,s[0]);
      int maxlen=1;
      if(len<2)
         return s;
   
      for(int i=0;i<len-1;i++)
      {
          string p1=helper(s,i,i);
          if (p1.size()>maxlen)
          {
               s1=p1;//奇数回文数
               maxlen=p1.size();
          }
               
         string p2=helper(s,i,i+1);
          if (p2.size()>maxlen)
          {
               s1=p2;//偶数回文数
               maxlen=p2.size();
          }
      }
      return s1;
    }
    string helper(string s,int c1,int c2)//中心扩展法，定义中心位置c1、c2
      {
          int i=c1,j=c2;
          while(i>=0&&j<s.size()&&s[i]==s[j])
          {
              i--;
              j++;
          }
          return s.substr(i+1,j-i-1);//(j-1)-(i+1)+1
          
      }
    
};