Given a string s, return the longest palindromic substring in s.

Example 1:

Input: s = "babad"
Output: "bab"
Note: "aba" is also a valid answer.
Example 2:

Input: s = "cbbd"
Output: "bb"
Example 3:

Input: s = "a"
Output: "a"
Example 4:

Input: s = "ac"
Output: "a"

Constraints:

1 <= s.length <= 1000
s consist of only digits and English letters (lower-case and/or upper-case)

public class Solution
{
	public string LongestPalindrome(string s){
		if(String.IsNullOrEmpty(s)){
			return s;
		}
		string longest=s.Substring(0,1);
		for(int i=0;i<s.Length;++i){
			string temp = used(s,i,i);
			if(temp.Length>longest.Length){
				longest = temp;
			}
			temp=used(s,i,i+1);
			if(temp.Length>longest.Length){
				longest = temp;
			}
		}
		return longest;
	}
	public string used(string s, int star, int end){
		while(star>=0&&end<s.Length&&s[star]==s[end]){
			--star;
			++end;
		}
		return s.Substring(star+1,end-star-1);
	}
}