Find the largest palindrome made from the product of two n-digit numbers.

Since the result could be very large, you should return the largest palindrome mod 1337.

Example:

Input: 2

Output: 987

Explanation: 99 x 91 = 9009, 9009 % 1337 = 987

 

Note:

The range of n is [1,8].

 

输入一个n，找出两个n位数的乘积为回文数的数，要求最大

 

C++(496ms):

1 class Solution { 2 public: 3     int largestPalindrome(int n) { 4         if (n == 1) 5             return 9 ; 6         int upper = pow(10 , n) - 1; 7         int lower = pow(10 , n-1) ; 8         for(int i = upper ; i >= lower ; i--){ 9             long res = buildPalindrome(i) ;10             for(long j = upper ; res <= j*j ; j--){11                 if (res % j == 0 && res / j <= upper)12                     return res % 1337 ;13             }14         }15         return -1 ;16     }17     18     long buildPalindrome(int num){19         string s = to_string(num) ;20         reverse(s.begin() , s.end()) ;21         return stol(to_string(num) + s) ;22     }23 };