Longest Palindromic Substring

 1 class Solution {
 2 public:
 3     string longestPalindrome(string s) {
 4         int n=s.size();
 5         if(n<2) return s;
 6         if(n==2&&(s[0]==s[1])) return s;
 7         
 8         int maxlength=0;
 9         int left=0;
10         int right=0;
11         string res;
12         for(int i=0;i<n-1;i++)
13         {
14             if(s[i]!=s[i+1]&&s[i]!=s[i+2])
15                 continue;
16             if(s[i]==s[i+1])
17             {
18                 left=i;
19                 right=i+1;
20             
21             
22                 while(left>0&&right<n-1)           
23                 {
24                     if(s[left-1]==s[right+1])
25                     {
26                         left--;
27                         right++;
28                         continue;
29                     }
30                     break;
31                 }
32                 int tmplength=right-left+1;
33                 if(tmplength>maxlength)
34                 {
35                     res=s.substr(left,tmplength);
36                     maxlength=tmplength;
37                 }
38             }
39             if(i+2<n&&s[i]==s[i+2])
40             {
41                 left=right=i+1;
42             
43                 while(left>0&&right<n-1)         //注意边界选取，若采用s[left]==s[right],则会出现abcbe res=abcbe的情况。
44                 {
45                     if(s[left-1]==s[right+1])
46                     {
47                         left--;
48                         right++;
49                         continue;
50                     }
51                     
52                     break;
53                 }
54                 int tmplength=right-left+1;
55                 if(tmplength>maxlength)
56                 {
57                     res=s.substr(left,tmplength);
58                     maxlength=tmplength;
59                 }
60             }
61         }
62         
63        
64         return res;
65     }
66 };