You’re in a coding interview. The interviewer says: “Traverse an m × n matrix in spiral order.” There’s no clever algorithm — just careful boundary management. You shrink your traversal window after each pass (right, down, left, up) and stop when the boundaries cross.

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The Problem

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

Example

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

Already comfortable with the solution? Practice it in a mock interview →

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What Interviewers Are Actually Testing

Skill	What Interviewers Observe
Boundary tracking	Do you track top, bottom, left, right and shrink them?
Loop termination	Do you check boundaries before left/up passes?
Non-square matrix	Does your solution handle m ≠ n, single row, single column?

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Step 1: The Key Insight

Maintain four boundaries: top, bottom, left, right. After traversing each edge, shrink the corresponding boundary inward. Check boundaries before the left and up passes to handle single-row or single-column remainders.

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Python Implementation

def spiralOrder(matrix: list[list[int]]) -> list[int]:
    result = []
    top, bottom = 0, len(matrix) - 1
    left, right = 0, len(matrix[0]) - 1

    while top <= bottom and left <= right:
        for c in range(left, right + 1):
            result.append(matrix[top][c])
        top += 1

        for r in range(top, bottom + 1):
            result.append(matrix[r][right])
        right -= 1

        if top <= bottom:
            for c in range(right, left - 1, -1):
                result.append(matrix[bottom][c])
            bottom -= 1

        if left <= right:
            for r in range(bottom, top - 1, -1):
                result.append(matrix[r][left])
            left += 1

    return result

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Time & Space Complexity

Aspect	Complexity
Time	O(m × n) — every cell visited once
Space	O(1) extra (not counting output)

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Common Interview Mistakes

1. Not checking boundaries before left/up passes. If only one row remains after the right pass, the left pass duplicates cells.
2. Off-by-one in ranges. range(left, right + 1) for right, range(right, left - 1, -1) for left.
3. Hardcoding for square matrices. Must handle m ≠ n. Test with 1×4 and 4×1.

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What a Strong Candidate Sounds Like

“Four boundary variables: top, bottom, left, right. Each iteration traverses one layer: right, down, left, up. After each pass I shrink the boundary. I guard left/up passes with boundary checks for single-row/column cases. O(m×n) time, O(1) extra space.”

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Related Problems to Practice

• Product of Array Except Self — Array traversal discipline.
• Binary Tree Level Order Traversal — Layer-by-layer traversal.

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Practice This in a Mock Interview

Start a mock interview for Spiral Matrix on Intervu.

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Further reading:

• How to Prepare for a Coding Interview in 2026, the complete roadmap
• Why LeetCode alone isn’t enough, and what to practice instead
• Practice any LeetCode problem as a live mock interview
• The Grind 75 Pathway, a structured study plan with AI practice links
• Browse all walkthroughs on GitHub