LeetCode-in-Cpp

73. Set Matrix Zeroes

Medium

Given an m x n integer matrix matrix, if an element is 0, set its entire row and column to 0’s, and return the matrix.

You must do it in place.

Example 1:

Input: matrix = [[1,1,1],[1,0,1],[1,1,1]]

Output: [[1,0,1],[0,0,0],[1,0,1]]

Example 2:

Input: matrix = [[0,1,2,0],[3,4,5,2],[1,3,1,5]]

Output: [[0,0,0,0],[0,4,5,0],[0,3,1,0]]

Constraints:

• m == matrix.length
• n == matrix[0].length
• 1 <= m, n <= 200
• -231 <= matrix[i][j] <= 231 - 1

Follow up:

• A straightforward solution using O(mn) space is probably a bad idea.
• A simple improvement uses O(m + n) space, but still not the best solution.
• Could you devise a constant space solution?

Solution

#include <vector>

class Solution {
public:
    void setZeroes(std::vector<std::vector<int>>& matrix) {
        int m = matrix.size();
        int n = matrix[0].size();
        bool row0 = false;
        bool col0 = false;

        // Check if 0th column needs to be marked all 0s in the future
        for (int i = 0; i < m; i++) {
            if (matrix[i][0] == 0) {
                col0 = true;
                break;
            }
        }

        // Check if 0th row needs to be marked all 0s in the future
        for (int j = 0; j < n; j++) {
            if (matrix[0][j] == 0) {
                row0 = true;
                break;
            }
        }

        // Store the signals in the 0th row and column
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (matrix[i][j] == 0) {
                    matrix[i][0] = 0;
                    matrix[0][j] = 0;
                }
            }
        }

        // Mark 0 for all cells based on signals from the 0th row and 0th column
        for (int i = 1; i < m; i++) {
            for (int j = 1; j < n; j++) {
                if (matrix[i][0] == 0 || matrix[0][j] == 0) {
                    matrix[i][j] = 0;
                }
            }
        }

        // Set 0th column
        if (col0) {
            for (int i = 0; i < m; i++) {
                matrix[i][0] = 0;
            }
        }

        // Set 0th row
        if (row0) {
            for (int j = 0; j < n; j++) {
                matrix[0][j] = 0;
            }
        }
    }
};

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