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"

 1 public class Solution {
 2     public string LongestPalindrome(string
 
 s) {
 3         if (s.Length < 2) return s;
 4         
 5         string curRes = "";
 6         
 7         for (int i = 0; i < s.Length; i++)
 8         {
 9             var tmp = findLongestPalindrome(s, 0, i, Math.Min(2 * i, s.Length - 1), true);
10             if (tmp.Length > curRes.Length) 
 
11             {
12                 curRes = tmp;
13             }
14             
15             tmp = findLongestPalindrome(s, 0, i, Math.Min(2 * i + 1, s.Length - 1), false);
16             if (tmp.Length > curRes.Length) 
17             {
18                 curRes = tmp;
19             }
20         }
 
21         
22         return curRes;
23     }
24     
25     private string findLongestPalindrome(string s, int start, int mid, int end, bool isCenter)
26     {
27 
28         int i = 0, j = 0;
29         if (isCenter)
30         {
31             i = mid - 1;
32             j = mid + 1;
33             
34             while (i >= start && j <= end)
35             {
36                 if (s[i] == s[j])
37                 {
38                     i--;
39                     j++;
40                 }
41                 else 
42                 {
43                     break;
44                 }
45             }
46             
47             return s.Substring(i + 1, j - i - 1);
48         }
49         else
50         {
51             i = mid;
52             j = mid + 1;
53             
54             while (i >= start && j <= end)
55             {
56                 if (s[i] == s[j])
57                 {
58                     i--;
59                     j++;
60                 }
61                 else 
62                 {
63                     break;
64                 }
65             }
66             
67             if (j - i - 1 < 1) return "";
68             
69             return s.Substring(i + 1, j - i - 1);
70         }
71     }
72 }

Leetcode 5: Longest Palindromic Substring