LeetCode #479 - Largest Palindrome Product

题目描述：

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].

找出由两个n位数相乘得到的最长的回文数，n最大为8，所以两个数的乘积不会超过long long变量，所以我们应该从位数为2n的最大回文数开始向前遍历，判断当前的回文数能够整除一个n位数。

class Solution {
public:
    int largestPalindrome(int n) {
        if(n==1) return 9;
        long long max_num=pow(10,n)-1;
        for(long long i=max_num;i>max_num/10;i--)
        {
            long long x=get_palindrome(i);
            for(long long j=max_num;j*j>=x;j--)
                if(x%j==0) return (x%1337);
        }
    }
    
    long long get_palindrome(int num)
    {
        string s=to_string(num);
        string result=s;
        reverse(s.begin(),s.end());
        result+=s;
        return atoll(result.c_str());
    }
};