Set Matrix Zeros

JavaView on GFG

Time: O(n*m)

Space: O(1)

Intuition

Set entire row and column to 0 if any element is 0. In-place O(1) space using first row/col as markers.

Algorithm

1First pass: mark which rows and columns need zeroing using first row and col. Second pass: zero marked rows/cols.

Common Pitfalls

•Same as LC 73. Track if first row/col themselves contain zero separately. O(mn) time O(1) space.

Set Matrix Zeros.java

Java

// Approach: Use first row/column as flags to mark which rows/columns should be zeroed; zero them in second pass.
// Time: O(n*m) Space: O(1)
class Solution {
    public void setMatrixZeroes(int[][] mat) {
        int n = mat.length;
        int m = mat[0].length;

        int col0 = 1;
        // step 1: Traverse the matrix and
        // mark 1st row & col accordingly:
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < m; j++) {
                if (mat[i][j] == 0) {
                    // mark i-th row:
                    mat[i][0] = 0;

                    // mark j-th column:
                    if (j != 0)
                        mat[0][j] = 0;
                    else
                        col0 = 0;
                }
            }
        }

        // Step 2: Mark with 0 from (1,1) to (n-1, m-1):
        for (int i = 1; i < n; i++) {
            for (int j = 1; j < m; j++) {
                if (mat[i][j] != 0) {
                    // check for col & row:
                    if (mat[i][0] == 0 || mat[0][j] == 0)
                        mat[i][j] = 0;
                }
            }
        }

        // step 3: Finally mark the 1st col & then 1st row:
        if (mat[0][0] == 0) {
            for (int j = 0; j < m; j++)
                mat[0][j] = 0;
        }
        if (col0 == 0) {
            for (int i = 0; i < n; i++)
                mat[i][0] = 0;
        }
    }
}

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