题解 | #火车进站#

# 使用深度优先搜，递归的出栈，直到站内与待入的都没有火车了，将结果记录
def dfs(nums, stack, temp, res):
    if not stack and not nums:
        res.append(' '.join(temp))
        return 

    if stack:
        temp.append(stack.pop())
        dfs(nums, stack, temp, res)
        stack.append(temp.pop())
    
    if nums:
        stack.append(nums.pop(0))
        dfs(nums, stack, temp, res)
        nums.insert(0,stack.pop())

N = int(input())
nums = input().split()
temp = []
res = []
stack = []
dfs(nums,stack,temp,res)
res.sort()

for r in res:
    print(r)