5. Longest Palindromic Substring(最长回文子串 manacher 算法)

Posted 2020-10-24 张乐乐章

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 class Solution:
 2     def longestPalindrome(self, s):
 3         """
 4         :type s: str
 5         :rtype: str
 6         """
 7         # preprocess
 8         slist = list(s)
 9         for i in range((len(s) + 1) * 2)[::2]:
10             slist.insert(i, "#")
11         slist.insert(0, ‘$‘)
12         
13         p = self.manacher(slist)
14         
15         i = p.index(max(p))
16         ans = ‘‘.join(slist[i-p[i]+1:i+p[i]])
17         return ans.replace(‘#‘,‘‘).replace(‘$‘,‘‘)
18     def manacher(self, slist):
19    
20 
21         # 计算p
22         p = [0] * len(slist)
23         p[0] = 1
24         id = 0
25         mx = 1
26         print(slist)
27         for i in range(1,len(slist)):
28             if mx > i:
29                 p[i] = min(p[id * 2 - i], mx - i)
30             else:
31                 p[i] = 1
32 
33             #暴力
34             while i +p[i]<len(slist) and slist[i - p[i]]==slist[i + p[i]]:
35                 p[i] = p[i]+1
36             #更新最大三元组
37             if(mx < i + p[i]):
38                 mx = i + p[i]
39                 id = i
40 
41         return p