Java [leetcode 5] 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, and there exists one unique longest palindromic substring.

解题思路：

遍历方法，时间复杂度为O(n)。首先从字符串的开头位置一直往后遍历，在每次遍历的过程中由该位置向两边扩散，直到找到最长的子回文串为止。同时需要考虑奇字符串和偶字符串的情况。

代码如下：

 1 public class Solution {
 2    public String longestPalindrome(String s) {
 3         int n = s.length();
 4         String longest = s.substring(0, 1);
 5
 6         if (s == null || s.length() == 0)
 7             return null;
 8         for (int i = 0; i < n; i++) {
 9             String p1 = expandFromCenter(s, i, i);
10             if (p1.length() > longest.length())
11                 longest = p1;
12             String p2 = expandFromCenter(s, i, i + 1);
13             if (p2.length() > longest.length())
14                 longest = p2;
15         }
16         return longest;
17     }
18
19     public String expandFromCenter(String s, int c1, int c2) {
20         int head = c1;
21         int tail = c2;
22         int m = s.length();
23
24         while (head >= 0 && tail < m && s.charAt(head) == s.charAt(tail)) {
25             head--;
26             tail++;
27         }
28         return s.substring(head + 1, tail);
29     }
30 }