gist for leetcode 479

largestPalindromeProduct.cpp

class Solution {
public:
    int largestPalindrome(int n) {
        if (n == 1) {
            return 9;
        }
        
        const int upperBound = static_cast<int>(pow(10.0, (double)n)) - 1;
        const int lowerBound = upperBound / 10;
        
        long palindrome;
        string numStr, reverseNumStr;
        
        for(int num = upperBound; num > lowerBound; num--) {
            numStr = to_string(num);
            reverseNumStr = to_string(num);
            reverse(reverseNumStr.begin(), reverseNumStr.end());
    
            palindrome = stol(numStr + reverseNumStr);
            
            //>> check the palindrome is the product of two n-digit number or not
            for(long p = upperBound; p*p >= palindrome; p--) {
                if(palindrome % p == 0) {
                    return static_cast<int>(palindrome % 1337);
                }
            }
        }
        
        return 0;
    }
};