I've seen this before so I wasn't panicking, but I still fumbled the boundary shrinking logic midway through.
Use boundary pointers (top, bottom, left, right) to traverse the matrix in four directions, shrinking the boundaries after each pass. Continue until all elements are visited, handling edge cases like single row or column.
Pro tip: Clarify edge cases upfront (empty matrix, single row/column) and discuss time/space complexity (O(m*n) time, O(1) extra space) to show thoroughness. Mention that this approach is optimal and commonly used in production code.
Restate the problem to ensure clarity. Identify edge cases: empty matrix, single row, single column, and non-square matrices.
Initialize four pointers: top=0, bottom=m-1, left=0, right=n-1. Plan to traverse right along top, down along right, left along bottom, and up along left, adjusting boundaries after each direction.
While top <= bottom and left <= right, perform the four directional traversals. After each traversal, update the corresponding boundary (e.g., top++ after moving right).
After the loop, ensure all elements are added. For odd dimensions, the last element may be added in the final traversal; check conditions to avoid duplicates.
State time complexity O(m*n) and space complexity O(1) excluding output. Walk through a small example to verify correctness.
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