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"

題意：找到一個字符串的最長回文子串

思路：枚舉中心位置，向兩邊擴展，並及時更新最長回文字串。此處回文長度是奇數和偶數分開處理。

 1 class Solution:
 
 2     def longestPalindrome(self,s):
 3         n = len(s)
 4         a = ‘‘
 5         if n < 1:
 6             return 0
 7         max = 0
 8         for i in range(n):   #i記錄回文串的中心
 9             j = 0
10             while (i-j >= 0) and (i+j < n):  #回文串長度為奇數
11                 if s[i-j] != s[i+j]:
 
12                     break
13                 c = j*2+1
14                 j += 1
15             if c > max:
16                 max = c
17                 a = s[i-j+1:i+j]
18             k = 0
19             while (i - k >= 0) and (i + k + 1 < n):  #回文串長度為偶數
20                 if s[i-k] != s[i+k+1]:
 
21                     break
22                 c = k*2+2
23                 k += 1
24             if c > max:
25                 max = c
26                 a = s[i - k + 1 : i + k + 1]
27         return a
28 
29 if __name__==‘__main__‘:
30     solution=Solution()
31     print(solution.longestPalindrome(‘abaab‘))

Leetcode:5- Longest Palindromic Substring