categories: leetcode

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题目描述

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.
Example 1: Input: “babad” Output: “bab” Note: “aba” is also a valid answer.Example 2: Input: “cbbd” Output: “bb”

参考代码

  1. class Solution {
  2.     public String longestPalindrome(String s) {
  3.         if (s == null || s.length() < 1) return "";
  4.         int start = 0, end = 0;
  5.         for (int i = 0; i < s.length(); i++) {
  6.             int len1 = expandAroundCenter(s, i, i);//奇数
  7.             int len2 = expandAroundCenter(s, i, i + 1);//偶数
  8.             int len = Math.max(len1, len2);
  9.             if (len > end - start) {
  10.                 start = i - (len - 1) / 2;//确使奇偶行得通
  11.                 end = i + len / 2;
  12.             }
  13.         }
  14.         return s.substring(start, end + 1);
  15.     }
  17.     private int expandAroundCenter(String s, int left, int right) {
  18.         int L = left, R = right;
  19.         while (L >= 0 && R < s.length() && s.charAt(L) == s.charAt(R))
  20.         {
  21.             L--;
  22.             R++;
  23.         }
  24.         return R - L - 1;//因为循环结束时，长度各边减一
  25.     }
  27. }

思路及总结

偶数减一除以二(0.5-1.0)和奇数减一除以二(0.0-0.5)两者可以保持除法上的一致。
官方首先提醒了最长公共子串的误区，然后列举了暴力法，动态规划，和本题的中心扩展法。详细内容直接参考官方题解。

参考

https://leetcode-cn.com/problems/longest-palindromic-substring/solution/
https://leetcode.com/problems/longest-palindromic-substring/solution/
Manacher 算法
https://segmentfault.com/a/1190000008484167