How do you traverse a matrix in spiral order?

def spiral_order(matrix):
    if not matrix: return []
    top, bottom = 0, len(matrix) - 1
    left, right = 0, len(matrix[0]) - 1
    res = []
    while top <= bottom and left <= right:
        for c in range(left, right + 1): res.append(matrix[top][c])
        top += 1
        for r in range(top, bottom + 1): res.append(matrix[r][right])
        right -= 1
        if top <= bottom:
            for c in range(right, left - 1, -1): res.append(matrix[bottom][c])
            bottom -= 1
        if left <= right:
            for r in range(bottom, top - 1, -1): res.append(matrix[r][left])
            left += 1
    return res


# --- demo ---
print(spiral_order([[1, 2, 3], [8, 9, 4], [7, 6, 5]]))
# [1, 2, 3, 4, 5, 6, 7, 8, 9]