最长回文（倒过来一样）子串（连续）

#include <iostream>
using namespace std;
#define N 1000

bool f

;   //f[i][j]表示[i,j]是否是回文子串

string longestPalindrome(string s)
{
    int n = s.size();
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            f[i][j] = false;
        }
    }
    int maxLen = 1, start = 0;  //子串长度，起点
    for (int i = 0; i < n; i++)
    {
        f[i][i] = true; //单个字符是回文子串
        for (int j = 0; j < i; j++) //[j,i]是不是回文子串
        {
            f[j][i] = (s[j] == s[i] && (i-j < 2 || f[j+1][i-1]));   
            if (f[j][i] && maxLen < (i - j + 1))    //i-j+1是[j,i]的长度
            {
                maxLen = i - j + 1;
                start = j;
            }
        }
    }
    return s.substr(start, maxLen);
}

int main()
{
    
    string s = "aaabcdcbabcdaa";
    cout << longestPalindrome(s).c_str() << endl;
    return 0 ;
}