Longest Palindromic Substring

 1 class Solution {
 2 public:
 3     string longestPalindrome(string s){
 4       int len=s.size();
 5       string s1(1,s[0]);
 6       int maxlen=1;
 7       if(len<2)
 8          return s;
 9    
10       for(int i=0;i<len-1;i++)
11       {
12           string p1=helper(s,i,i);
13           if (p1.size()>maxlen)
14           {
15                s1=p1;//奇数回文数
16                maxlen=p1.size();
17           }
18                
19          string p2=helper(s,i,i+1);
20           if (p2.size()>maxlen)
21           {
22                s1=p2;//偶数回文数
23                maxlen=p2.size();
24           }
25       }
26       return s1;
27     }
28     string helper(string s,int c1,int c2)//中心扩展法，定义中心位置c1、c2
29       {
30           int i=c1,j=c2;
31           while(i>=0&&j<s.size()&&s[i]==s[j])
32           {
33               i--;
34               j++;
35           }
36           return s.substr(i+1,j-i-1);//(j-1)-(i+1)+1
37           
38       }
39     
40 };