leetcode 479. Largest Palindrome Product

Description

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].

Solution

从合适位置开始, 合适位置结束, 暴力搜索

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 long cand = creatPalindrome(i);
            for (int j = upper; cand / j < j; --j) {
                if (cand % j == 0) return cand % 1337;
            }
        }
        return -1;
    }

private:
    long long creatPalindrome(int n) {
        string lastHalf = to_string(n);
        reverse(lastHalf.begin(), lastHalf.end());
        return stoll(to_string(n) + lastHalf);
    }
};

Reference

• leetcode 479. Largest Palindrome Product
• stol
• stoll