题解 | #火车进站#

import sys
import copy


num = int(sys.stdin.readline())
line = sys.stdin.readline()
stack = line.split()
input_li = []
output_li = []
ret_li = []


def compute(stack, input_li, output_li):
    global ret_li
    if len(output_li) >= num:
        ret_li.append(copy.copy(output_li))
        return
    
    # 只出站
    if input_li:
        output_li.append(input_li.pop())
        compute(stack, input_li, output_li)
        input_li.append(output_li.pop())
    
    # 只进站
    if stack:
        input_li.append(stack.pop(0))
        compute(stack, input_li, output_li)
        stack.insert(0, input_li.pop())


compute(stack, input_li, output_li)
ret_li.sort()
for ret in ret_li:
    print(' '.join(ret))