Leetcode:5- Longest Palindromic Substring

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"

题意：找到一个字符串的最长回文子串

思路：枚举中心位置，向两边扩展，并及时更新最长回文字串。此处回文长度是奇数和偶数分开处理。

class Solution:
    def longestPalindrome(self,s):
        n = len(s)
        a = ''
        if n < 1:
            return 0
        max = 0
        for i in range(n):   #i记录回文串的中心
            j = 0
            while (i-j >= 0) and (i+j < n):  #回文串长度为奇数
                if s[i-j] != s[i+j]:
                    break
                c = j*2+1
                j += 1
            if c > max:
                max = c
                a = s[i-j+1:i+j]
            k = 0
            while (i - k >= 0) and (i + k + 1 < n):  #回文串长度为偶数
                if s[i-k] != s[i+k+1]:
                    break
                c = k*2+2
                k += 1
            if c > max:
                max = c
                a = s[i - k + 1 : i + k + 1]
        return a

if __name__=='__main__':
    solution=Solution()
    print(solution.longestPalindrome('abaab'))