Spiral Matrix

Given an R x C matrix, return every element in spiral order — starting top-left, moving right, then down, then left, then up, shrinking inward each lap.

Example

Input:

Output: 1 2 3 6 9 8 7 4 5

Approach

Track four shrinking boundaries — top, bottom, left, right. Each lap: walk the top row left-to-right (then push top down), the right column top-to-bottom (then pull right in), the bottom row right-to-left if a bottom row still remains (then pull bottom up), and the left column bottom-to-top if a left column still remains (then push left in). The two "if" guards matter — without them, a single remaining row or column gets walked twice once the boundaries cross.

Walkthrough

1def spiral_matrix(grid):
2    top, bottom = 0, len(grid) - 1
3    left, right = 0, len(grid[0]) - 1
4    result = []
5    while top <= bottom and left <= right:
6        for col in range(left, right + 1):
7            result.append(grid[top][col])
8        top += 1
9        for row in range(top, bottom + 1):
10            result.append(grid[row][right])
11        right -= 1
12        if top <= bottom:
13            for col in range(right, left - 1, -1):
14                result.append(grid[bottom][col])
15            bottom -= 1
16        if left <= right:
17            for row in range(bottom, top - 1, -1):
18                result.append(grid[row][left])
19            left += 1
20    return result

Input format

Line 1: R C Next R lines: C space-separated integers

Print the spiral order, space-separated.