Longest Palindromic Substring

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.

思路：找出字符串s中最长的回文子串。我们可以这样考虑，从中间向两边扩展，尽可能找出最长的回文子串。在此注意要考虑奇偶位置的比较，这一点要考虑清楚。

class Solution {
public:
    string Sub_Palindrome(string s,int left,int right)
    {
        int l=left,r=right;
        int n=s.length();
        while(l>=0 && r<=n-1 && s[l]==s[r])
        {
            l--;
            r++;
        }
        return s.substr(l+1,r-l-1);
    }
    string longestPalindrome(string s) {
        int n=s.length();
        if(n==0)
            return "";
        string longest=s.substr(0,1);
        for(int i=0;i<n-1;i++)
        {
            string sub_str1=Sub_Palindrome(s,i,i);//奇数
            if(sub_str1.length()>longest.length())
                longest=sub_str1;
            string sub_str2=Sub_Palindrome(s,i,i+1);//偶数
            if(sub_str2.length()>longest.length())
                longest=sub_str2;
        }
        return longest;
    }
};