1253. Reconstruct a 2-Row Binary Matrix

Given the following details of a matrix with n columns and 2 rows :

• The matrix is a binary matrix, which means each element in the matrix can be 0 or 1.
• The sum of elements of the 0-th(upper) row is given as upper.
• The sum of elements of the 1-st(lower) row is given as lower.
• The sum of elements in the i-th column(0-indexed) is colsum[i], where colsum is given as an integer array with length n.

Your task is to reconstruct the matrix with upper, lower and colsum.

Return it as a 2-D integer array.

If there are more than one valid solution, any of them will be accepted.

If no valid solution exists, return an empty 2-D array.

class Solution {
public:
    vector<vector<int>> reconstructMatrix(int upper, int lower, vector<int>& colsum) {
        int r1 = 0, r2 = 0, n = colsum.size();
        vector<vector<int>> res(2, vector<int>(n, 0));
        for(int i = 0; i < n; i++) {
            if(colsum[i] == 2) {
                res[0][i] = res[1][i] = 1;
                r1 += 1;
                r2 += 1;
            }
        }
        for(int i = 0; i < n; i++) {
            if(colsum[i] == 1) {
                if(r1 < upper) {
                    res[0][i] = 1;
                    r1 += 1;
                } else {
                    res[1][i] = 1;
                    r2 += 1;
                }
            }
        }
        if(r1 != upper or r2 != lower) return {};
        return res;
    }
};