LeetCode（Python3）5.最长回文子串

Question：

Given a string s, return the longest palindromic substring in s.

Example1：

Input: s = "babad"

Output: "bab"

Explanation: "aba" is also a valid answer.

Example2:

Input: s = "cbbd"

Output: "bb"

Mentality:

这道题可以用中心扩散法来解，依次判断我们子串所能达到的最远边距。但是要注意：我们的子串分为奇数长度和偶数长度两种情况，根据这两种情况，我们要做不同的处理。

Code:

class Solution:
    def longestPalindrome(self, s: str) -> str:
        start = 0
        end = 0
        for i in range(len(s)):
            left1, right1 = self.expendaroundcenter(s, i, i)
            left2, right2 = self.expendaroundcenter(s, i, i+1)
            if right1 - left1 > end - start:
                start, end = left1, right1
            if right2 - left2 > end - start:
                start, end = left2, right2
        return s[start: end + 1]
        
    def expendaroundcenter(self, s, left, right):
        while left >= 0 and right < len(s) and s[left] == s[right]:
            left -= 1
            right += 1
        return left + 1, right - 1

Result:

---

可以发现，上述结果还存在可以优化的空间，于是我去学习了一下大佬们的代码，在这分享

Code:

class Solution:
    def longestPalindrome(self, s: str) -> str:
        res = ''
        for i in range(len(s)):
            start = max(0, i-len(res)-1)
            temp = s[start:i+1]
            if temp == temp[::-1]:
                res = temp
            else:
                temp = temp[1:]
                if temp == temp[::-1]:
                    res = temp
        return res

Result：

 快了将近十倍。。但是我感觉思路差不多，他的判断很巧妙。