n,head=map(int,input().split())
a=[]
for i in range(n):
    x,y=map(int,input().split())
    a.append([x,y])
p=head
while p!=-1:
    print(a[p][0],end=" ")
    p=a[p][1]

class Node:
    def __init__(self,val:int,nxt:int):
        self.val=val
        self.nxt=nxt
def add (it,val):
    a.append(Node(val,a[it].nxt))
    a[it].nxt=len(a)-1
def delete (it):
    a[it].nxt=a[a[it].nxt].nxt
a=[Node(0,0) for i in range(3000)]
n,h=map(int,input().split())
for i in range(n):
    a[i].val,a[i].nxt=map(int,input().split())
it=h
while it!=-1:
    print(a[it].val,end=" ")
    it=a[it].nxt

题解qp 代码仅供参考，课上讲的实现方式还是要会的，class写法较好写好理解