73. Set Matrix Zeroes

• Difficulty: Medium

• Topics: Array

• Similar Questions:

  • Game of Life

Problem:

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in-place.

Example 1:

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

Example 2:

Input: 
[
  [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]
]

Follow up:

• A straight forward 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?

Solutions:

class Solution {
public:
    void setZeroes(vector<vector<int>>& matrix) {
        int m = matrix.size();
        if (m == 0) return;
        int n = matrix[0].size();
        if (n == 0) return;

        bool firstRowSet = false;
        bool firstColSet = false;

        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (matrix[i][j] == 0) {
                    if (i == 0 && j == 0) {
                        firstRowSet = true;
                        firstColSet = true;
                    } else if (i == 0) {
                        firstRowSet = true;
                        matrix[0][j] = 0;
                    } else if (j == 0) {
                        firstColSet = true;
                        matrix[i][0] = 0;
                    } else {
                        matrix[i][0] = 0;
                        matrix[0][j] = 0;
                    }
                }
            }
        }


        for (int i = 1; i < m; ++i) {
            if (matrix[i][0] == 0) {
                for (int j = 1; j < n; ++j) {
                    matrix[i][j] = 0;
                }
            }
        }



        for (int j = 1; j < n; ++j) {
            if (matrix[0][j] == 0) {
                for (int i = 1; i < m; ++i) {
                    matrix[i][j] = 0;
                }
            }
        }

        if (firstRowSet) {
            for (int j = 0; j < n; ++j) {
                matrix[0][j] = 0;
            }
        }

        if (firstColSet) {
            for (int i = 0; i < m; ++i) {
                matrix[i][0] = 0;
            }
        }
    }
};