Spiral Matrix Traversal

The cleanest spiral traversal is boundary based. The current unvisited region is the rectangle [top..bottom] x [left..right]. Emit its top edge left to right, then its right edge top to bottom. Shrink both boundaries. If rows remain, emit the bottom edge right to left; if columns remain, emit the left edge bottom to top. Continue until the rectangle is empty. This representation makes the correctness argument simple: after each side is emitted, that side cannot contain an unvisited cell, so moving its boundary excludes exactly visited cells. Do not use a visited matrix unless the traversal can change direction around obstacles; for a plain rectangle it is unnecessary space. The algorithm is O(mn) time because every element is output once and O(1) auxiliary space excluding output. Test 1x1, 1xn, nx1, and rectangular input.