Spiral Matrix Traversal

Requirements

Given an $m \times n$ matrix, return one list containing all of its elements in clockwise spiral order. Begin by traversing the top row from left to right, continue down the rightmost column, traverse the bottom row from right to left, and then move up the leftmost column before repeating the process for the remaining inner matrix.

Implement the following operation:

spiral_order(matrix)

Examples

Example 1

Input:

[[1, 2, 3], [4, 5, 6]]

Output:

[1, 2, 3, 6, 5, 4]

The traversal visits the top row, the right edge, and then the bottom row in reverse order.

Example 2

Input:

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

Output:

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

After the outer ring is consumed, only the center value remains to be appended.

Example 3

Input:

[[1], [2], [3]]

Output:

[1, 2, 3]

A one-column matrix is read from top to bottom without revisiting any element.

Constraints

• The input is an $m \times n$ matrix.
• The traversal must also work for non-square matrices, including shapes such as $3 \times 5$ and $5 \times 3$.

Notes

• This is intended as a warm-up problem before the more difficult follow-up, Maximum-Sum Path in a Matrix, in the GSAM Infra interview loop.
• The expected solution time is under 10 minutes so that there is time for the second question.

Preparation

• Practice the edge cases on $3 \times 5$ and $5 \times 3$ matrices.
• LeetCode 54, “Spiral Matrix,” is the standard equivalent problem.