Formatted question description: https://leetcode.ca/all/1253.html

# 1253. Reconstruct a 2-Row Binary Matrix (Medium)

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.

Example 1:

Input: upper = 2, lower = 1, colsum = [1,1,1]
Output: [[1,1,0],[0,0,1]]
Explanation: [[1,0,1],[0,1,0]], and [[0,1,1],[1,0,0]] are also correct answers.


Example 2:

Input: upper = 2, lower = 3, colsum = [2,2,1,1]
Output: []


Example 3:

Input: upper = 5, lower = 5, colsum = [2,1,2,0,1,0,1,2,0,1]
Output: [[1,1,1,0,1,0,0,1,0,0],[1,0,1,0,0,0,1,1,0,1]]


Constraints:

• 1 <= colsum.length <= 10^5
• 0 <= upper, lower <= colsum.length
• 0 <= colsum[i] <= 2

Related Topics:
Math, Greedy

Similar Questions:

## Solution 1.

// OJ: https://leetcode.com/problems/reconstruct-a-2-row-binary-matrix/

// Time: O(N)
// Space: O(1)
class Solution {
public:
vector<vector<int>> reconstructMatrix(int upper, int lower, vector<int>& colsum) {
if (accumulate(begin(colsum), end(colsum), 0) != upper + lower) return {};
vector<vector<int>> ans(2, vector<int>(colsum.size()));
for (int i = 0; i < colsum.size(); ++i) {
if (colsum[i] == 2) --upper, --lower, ans[i] = ans[i] = 1;
if (upper < 0 || lower < 0) return {};
}
for (int i = 0; i < colsum.size(); ++i) {
if (colsum[i] != 1) continue;
if (upper) --upper, ans[i] = 1;
else --lower, ans[i] = 1;
if (upper < 0 || lower < 0) return {};
}
return ans;
}
};


Java

class Solution {
public List<List<Integer>> reconstructMatrix(int upper, int lower, int[] colsum) {
List<List<Integer>> matrix = new ArrayList<List<Integer>>();
int sum = 0;
int upperRemain = upper, lowerRemain = lower;
for (int num : colsum) {
sum += num;
if (num == 2) {
upperRemain--;
lowerRemain--;
}
}
if (sum != upper + lower || upperRemain < 0 || lowerRemain < 0)
return matrix;
for (int i = 0; i < 2; i++)
int columns = colsum.length;
for (int i = 0; i < columns; i++) {
int curSum = colsum[i];
if (curSum == 2) {
} else if (curSum == 1) {
if (upperRemain > 0) {
upperRemain--;
} else {
lowerRemain--;
}
} else {