def print_circle(matrix,up_hang,up_lie,down_hang,down_lie):
  result=[]
  
  if up_lie==down_hang and down_hang==down_lie: # 若只有一个元素
    result.append(matrix[up_hang][up_lie])
  elif up_lie==down_hang or up_lie==down_lie:  #若只有一行或一列元素
    if up_lie==down_hang:
      while up_lie <= down_lie:
        result.append(matrix[up_hang][up_lie])
        up_lie+=1
    elif up_lie==down_lie:
      while up_hang <=down_hang:
        result.append(matrix[up_hang][up_lie])
        up_hang+=1
# return result    #注意对齐方式，其决定了作用的区间范围，很关键    
    return result   
  i=up_hang
  j=up_lie
  while j<down_lie:
    result.append(matrix[i][j])
    j+=1
  while i<down_hang:
    result.append(matrix[i][j])
    i+=1
  while j>up_lie:
    result.append(matrix[i][j])
    j-=1
  while i>up_hang:
    result.append(matrix[i][j])
    i-=1
  return result

#matrix=[[1,2,3],[4,5,6],[7,8,9],[10,11,12],[13,14,15],[16,17,18]]
#matrix=[[1,2,3,4,5],[6,7,8,9,10],[11,12,13,14,15],[16,17,18,19,20],[21,22,23,24,25]]
matrix=[[1,2,3,4],[5,6,7,8],[9,10,11,12],[13,14,15,16]]
#matrix=[[1,2,3],[4,5,6],[7,8,9]]
#matrix=[[1,2],[3,4]]
re_mat=[]
up_hang=0
up_lie=0
down_hang=3
down_lie=3
if down_hang>down_lie:
  flag=down_lie
else:
  flag=down_hang
while flag!=0:       #flag决定了一个矩阵需要打印多少圈
  temp=print_circle(matrix,up_hang,up_lie,down_hang,down_lie)
  re_mat.extend(temp)
  up_hang+=1
  up_lie+=1
  down_hang-=1
  down_lie-=1
  flag=flag/2
print(re_mat)