leetcode5 Longest Palindromic Substring

 1 """
 2 Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
 3 Example 1:
 4 Input: "babad"
 5 Output: "bab"
 6 Note: "aba" is also a valid answer.
 7 Example 2:
 8 Input: "cbbd"
 9 Output: "bb"
10 """
11 """
12 用动态规划做
13 """
14 class Solution(object):
15     def longestPalindrome(self, s):
16         if s is None:
17             return ‘‘
18         ret = ‘‘
19         lens = len(s)
20         max = 0
21         dp = [[0] * lens for i in range(lens)]
22         #产生一个lens*lens全0二维数组，dp数组后面存储True或False
23         for j in range(lens):
24             for i in range(j + 1):
25                 dp[i][j] = ((s[i] == s[j]) and (j - i <= 2 or dp[i + 1][j - 1]))
26                 #如果s[i]=s[j]说明串的首尾相同，
27                 # 并且j-i为0表示只有一个字符必为回文，
28                 # j-i=1两个字符切相等必为回文，
29                 # j-i=2三个字符首尾相同无论中间是什么必为回文，
30                 # 或者dp[i + 1][j - 1]为真表示去掉首尾为回文，而新加的首尾相同必为回文。
31                 if dp[i][j] and j - i + 1 > max:
32                     max = j - i + 1
33                     ret = s[i:j + 1]
34                     #表示i开头j结束的串回文并且最长则更新长度max和回文串ret。
35         return ret