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：

public class Solution {
   public String longestPalindrome(String s) {
		int n = s.length();
		String longest = s.substring(0, 1);

		if (s == null || s.length() == 0)
			return null;
		for (int i = 0; i < n; i++) {
			String p1 = expandFromCenter(s, i, i);
			if (p1.length() > longest.length())
				longest = p1;
			String p2 = expandFromCenter(s, i, i + 1);
			if (p2.length() > longest.length())
				longest = p2;
		}
		return longest;
	}

	public String expandFromCenter(String s, int c1, int c2) {
		int head = c1;
		int tail = c2;
		int m = s.length();

		while (head >= 0 && tail < m && s.charAt(head) == s.charAt(tail)) {
			head--;
			tail++;
		}
		return s.substring(head + 1, tail);
	}
}