题解 | 火车进站

# Process input
n = int(input())
ax = input().split()

# DFS
def dfs(arr, i, station, seq, orders):
    if i == len(arr) and not station:
        orders.append(' '.join(seq))
        return
    if i < len(arr):
        station.append(arr[i])
        dfs(arr, i+1, station, seq, orders)
        station.pop()
    if station:
        train = station.pop()
        seq.append(train)
        dfs(arr, i, station, seq, orders)
        seq.pop()
        station.append(train)

res = []
dfs(ax, 0, [], [], res)

# Output
res.sort()
for row in res:
    print(row)