【LeetCode】#5最长回文子串(Longest Palindromic Substring)

【LeetCode】#5最长回文子串(Longest Palindromic Substring)

题目描述

给定一个字符串 s，找到 s 中最长的回文子串。你可以假设 s 的最大长度为 1000。

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

示例

示例1：

输入: “babad”
输出: “bab”
注意: “aba” 也是一个有效答案。

Input: “babad”
Output: “bab”
Note: “aba” is also a valid answer.

示例2：

输入: “cbbd”
输出: “bb”

Input: “cbbd”
Output: “bb”

解法

class Solution{
	public String longestPalindrome(String s){
		if(s==null || s.length()<1)
			return "";
		int start = 0, end = 0;
		for(int i=0; i<s.length(); i++){
			int len1 = expandAroundCenter(s, i, i);
			int len2 = expandAroundCenter(s, i, i+1);
			int len = Math.max(len1, len2);
			if(len>end-start){
				start = i - (len-1)/2;
				end = i + len/2;
			}
		}
		return s.substring(start, end+1);
	}
	private int expandAroundCenter(String s, int left, int right){
		int L = left, R = right;
		while(L>=0 && R<s.length() && s.charAt(L)==s.charAt(R)){
			L--;
			R++;
		}
		return R-L-1;
	}
}