3723. Maximize Sum of Squares of Digits

You are given two positive integers num and sum.

A positive integer n is good if it satisfies both of the following:

• The number of digits in n is exactly num.
• The sum of digits in n is exactly sum.

The score of a good integer n is the sum of the squares of digits in n.

Return a string denoting the good integer n that achieves the maximum score. If there are multiple possible integers, return the maximum one. If no such integer exists, return an empty string.

class Solution {
public:
    string maxSumOfSquares(int num, int sum) {
        if(num * 9 < sum) return "";
        string res = "";
        for(int i = 0; i < num; i++) {
            int now = min(sum, 9);
            res.push_back(now + '0');
            sum -= now;
        }
        return res;
    }
};