479. Largest Palindrome Product

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):

class Solution {
public:
    int largestPalindrome(int n) {
        if (n == 1)
            return 9 ;
        int upper = pow(10 , n) - 1;
        int lower = pow(10 , n-1) ;
        for(int i = upper ; i >= lower ; i--){
            long res = buildPalindrome(i) ;
            for(long j = upper ; res <= j*j ; j--){
                if (res % j == 0 && res / j <= upper)
                    return res % 1337 ;
            }
        }
        return -1 ;
    }
    
    long buildPalindrome(int num){
        string s = to_string(num) ;
        reverse(s.begin() , s.end()) ;
        return stol(to_string(num) + s) ;
    }
};