[LeetCode] 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].

 先找出符合位数的回文数，然后对回文数进行判断能否由两个n位数相乘。最后将符合条件的回文数整除。

class Solution {
public:
    int largestPalindrome(int n) {
        if (n == 1)
            return 9;
        long maxVal = pow(10, n) - 1;
        long minVal = maxVal / 10;
        for (long i = maxVal; i >= minVal; i--) {
            long val = createPalindrome(i);
            for (int j = maxVal; j >= minVal; j--) {
                if (val / j > maxVal) {
                    break;
                }
                if (val % j == 0) {
                    return static_cast<int>(val % 1337);
                }
            }
        }
        return 0;
    }
    
    long createPalindrome(long num) {
        long tmp = num;
        long a = 0;
        int n = 0;
        while(tmp) {
            a = a * 10 + tmp % 10;
            n++;
            tmp /= 10;
        }
        while(n) {
            num = num *10;
            n--;
        }
        return num + a;
    }
};
// 1354 ms

Trick algorithm：

class Solution {
public:
    int largestPalindrome(int n) {
        vector<long> pattern{9, 9009, 906609, 99000099, 9966006699, 999000000999, 99956644665999, 9999000000009999};
        return static_cast<int>(pattern[n - 1] % 1337);
    }
};
// 0 ms