Longest Palindromic Substring

以每一個，每兩個字符爲中心，求出最大對稱子串。返回所有子串中最長的子串， 時間複雜度O(N^2)。歡迎交流更佳算法。

class Solution {
public:
    string longestPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(s.size() <= 1) return s;
		int len = s.size();	
		int max = -1;
		int mid1 = -1, mid2 = -1;
		for(int i = 1; i < len; i ++)
		{
			int current_len = 0, start = i - 1, end = i;
			while(start >= 0 && end < len && s[start] == s[end])
			{
				current_len += 2;
				start --;
				end ++ ;
			}
			if(current_len > max)
			{
				max = current_len;
				mid1 = i - 1;
				mid2 = i;
			}
			current_len = 1, start = i - 1, end = i + 1;
			while(start >= 0 && end < len && s[start] == s[end])
			{
				current_len += 2;
				start --;
				end ++ ;
			}
			if(current_len > max)
			{
				max = current_len;
				mid1 = i;
				mid2 = -1;
			}
		}
		
		int begin;
		if(mid2 != -1)
		{
			begin = mid1 - max / 2 + 1;
		}
		else
		{
			begin = mid1 - max / 2;
		}
		string ans = s.substr(begin, max);
		return ans;
    }
};