How do you approach Set Matrix Zeroes?

Use the first row and first column as markers.

What is the time complexity of Set Matrix Zeroes?

Time complexity: O(m * n). Space complexity: O(1).

What are common mistakes on Set Matrix Zeroes?

Transposing row and column in the index. It reads fine and fails on any non-square input. Overwriting cells a later iteration still needs to read - most in-place matrix problems want a marker pass first. Bounds checks that cover only some of the four edges.

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Set Matrix Zeroes

MediumAlgorithms

Description

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

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 - -2^31 <= matrix[i][j] <= 2^31 - 1

Set Matrix Zeroes: approach, complexity and pitfalls

A medium matrix problem, graded against 5 test cases (2 of them hidden).

The pattern: Matrix

2D grid traversal, rotation, and in-place manipulation.

Reach for it when you see: A grid input, spiral or diagonal traversal, or an in-place transformation constraint.

If you are stuck

Show the 3-step hint ladder

Use the first row and first column as markers.
First pass: mark which rows and columns need to be zeroed.
Second pass: set the cells to zero based on the markers.

The optimal approach

Show the approach: Use First Row/Column as Markers

Use the first row and column to mark which rows/columns should be zeroed. Handle the first row/column separately.

Time: O(m * n)
Space: O(1)

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Single element. A one-element case is included. Two-pointer and window solutions frequently return the wrong thing when the two indices start out equal.
Duplicate values. Repeated values appear in the input, so any logic that assumes distinct elements - or that skips over equal neighbours - is exercised.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Matrix mistakes

These apply to the pattern as a whole, not just this problem.

Transposing row and column in the index. It reads fine and fails on any non-square input.
Overwriting cells a later iteration still needs to read - most in-place matrix problems want a marker pass first.
Bounds checks that cover only some of the four edges.