Spiral Matrix

Use four inclusive boundaries: top=0, bottom=rows-1, left=0, and right=cols-1. While top <= bottom and left <= right, traverse the top row from left to right and increment top; traverse the right column from top to bottom and decrement right. Before traversing the bottom row, ensure top <= bottom, then go right to left and decrement bottom. Before traversing the left column, ensure left <= right, then go bottom to top and increment left. Those two guards avoid revisiting a row in a one-row remainder or a column in a one-column remainder. Every cell is emitted once, so time is O(rows*cols) and auxiliary space is O(1) besides the output. State behavior for empty matrices before reading the first row, since matrix[0] is unsafe when there are zero rows.