LeetCode——005 Longest Palindromic Substring

Description

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"

** Solution: **

使用中将对称思路
及从头开始遍历，对于每个字母进行左右向外扩散，判断是不是回文子串
注意，向外扩散有两种方式，第一种，就是以该字母为中心向外扩散，第二种就是该字母和其右端的字母开始进行向外扩散

class Solution {
public:
	string longestPalindrome(string s) {
		string res = "";
		for (int i = 0; i < s.length(); ++i)
		{
			help(s, i - 1, i + 1, res);//奇数类型的中心向两边扩撒
			help(s, i, i + 1, res);//偶数类型的中心向两边扩撒			
		}
		return res;
	}
	void help(const string &s, int L, int R, string &res)
	{
		while (L >= 0 && R < s.length())
		{
			if (s[L] != s[R])break;
			--L;
			++R;
		}
		if (R - L - 1 > res.length())
			res = s.substr(L + 1, R - L - 1);
	}
};