题目：

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：

遍历整个字符串，针对每个字符从中间向两边计算对称字符串。计算对称字符串的时候要考虑奇数与偶数两种情况。

代码：

class Solution {
public:
    string longestPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        string max;
        for(int i = 0; i < s.size(); i++)
        {
            string str = palindrome(s, i);
            if(str.size() > max.size())
            {
                max = str;
            }
        }
        return max;
    }
    
    string palindrome(string s, int i)
    {
        string str;
        int low1 = i;
        int high1 = i;
        while(low1>=0 && high1 < s.size() && s[low1-1] == s[high1+1])
        {
            low1--;
            high1++;
        };
        
        
        int low2 = i-1;
        int high2 = i;
        while(low2>=0 && high2 < s.size() && s[low2] == s[high2])
        {
            low2--;
            high2++;
        };
        low2++;
        high2--;
        
        if(high1 - low1 >= high2 - low2)
        {
            for(int k = low1; k <= high1; k++)
                str.push_back(s[k]);
        }
        else
        {
            for(int k = low2; k <= high2; k++)
                str.push_back(s[k]);
        }
        return str;
    }
};