Leetcode5-Longest Palindromic Substring

问题描述

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

示例

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.
Example 2:

Input: "cbbd"
Output: "bb"

算法思路

两种方法均可解决该问题，其中，第一种方法比较复杂，但是效率更高，第二种方法比较容易理解，但是效率较低。运行时间比较（88 ms < 3472 ms）。

代码实现(Python3)

方法一：

class Solution:
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        if len(set(s)) == 1:
            return s
        elif s == s[::-1]: # s is already a palindrome
            return s
        
        Len   = 1
        start = 0
        for i in range(1, len(s)): # index values of string
            p1, p2 = i - Len, i + 1  
            if p1 >= 1:
                temp = s[p1 - 1 : p2]
                if temp == temp[::-1]:
                    start = p1 - 1
                    Len += 2
                    continue
            if p1 >= 0:
                temp = s[p1:p2]
                if temp == temp[::-1]:
                    start = p1
                    Len += 1
        return s[start:start + Len]

方法二：

class Solution:
    def longestPalindrome(self, s):
        """
        :type s: str
        :rtype: str
        """
        if len(s) == 0: return ""
        self.longestP = s[0]

        def expand(s, l, r):
            while l >= 0 and r < len(s):
                if s[l] == s[r]:
                    sub = s[l:r + 1]
                    if len(sub) > len(self.longestP):
                        self.longestP = sub
                    l = l - 1
                    r = r + 1
                else:
                    break

        for i in range(len(s)):
            l, r = i, i + 1
            expand(s, l, r) # to find even substrings
            l, r = i - 1, i + 1
            expand(s, l, r) # to find odd substrings

        return self.longestP

参考资料

1. https://leetcode.com/problems/longest-palindromic-substring/ longest-palindromic-substring
2. https://leetcode.com/problems/longest-palindromic-substring/discuss/172203/Python-Simple-Solution-O(n2)-time-and-O(1)-space Python - Simple Solution - O(n^2) time and O(1) space