def spiral_order(matrix):
if not matrix or not matrix[0]:
return []
res = []
top, bottom = 0, len(matrix) - 1
left, right = 0, len(matrix[0]) - 1
while True:
for i in range(left, right + 1):
res.append(matrix[top][i])
top += 1
if top > bottom:
break
for i in range(top, bottom + 1):
res.append(matrix[i][right])
right -= 1
if right < left:
break
for i in range(right, left - 1, -1):
res.append(matrix[bottom][i])
bottom -= 1
if bottom < top:
break
for i in range(bottom, top - 1, -1):
res.append(matrix[i][left])
left += 1
if left > right:
break
return res
