How do you approach Spiral Matrix?

Use four pointers: top, bottom, left, right.

What is the time complexity of Spiral Matrix?

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

What are common mistakes on Spiral Matrix?

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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Spiral Matrix

MediumAlgorithms

Description

Given an `m x n` `matrix`, return all elements of the `matrix` in spiral order.

Example 1

Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]

Output: [1,2,3,6,9,8,7,4,5]

Example 2

Input: matrix = [[1,2,3,4],[5,6,7,8],[9,10,11,12]]

Output: [1,2,3,4,8,12,11,10,9,5,6,7]

Constraints

- m == matrix.length - n == matrix[i].length - 1 <= m, n <= 10 - -100 <= matrix[i][j] <= 100

Spiral Matrix: approach, complexity and pitfalls

A medium matrix problem, graded against 6 test cases (3 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.

More Matrixproblems →

If you are stuck

Show the 3-step hint ladder

Use four pointers: top, bottom, left, right.
Traverse right, down, left, up, then shrink the boundaries.
Continue until all elements are visited.

The optimal approach

Show the approach: Layer by Layer

Use four boundaries (top, bottom, left, right). Traverse each layer in spiral order, then shrink boundaries.

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

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.

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.