可持续化栈

#include <bits/stdc++.h>
using namespace std;
struct Stack{
	int n,next;
}s[10000];
vector<int> sav;
int size=0,top=0;
void push(int a){
	size++;
	s[size].n=a;
	s[size].next=top;
	top=size;
}
void pop(){
	top=s[top].next;
}
void out(){
	for(int i=top;i;i=s[i].next)
		cout<<s[i].n<<‘ ‘;
	puts("");
}
void open(int a){
	top=sav[a];
}
void save(){
	sav.push_back(top);
}
int main(){
	while(1){
		int a;
		cin>>a;
		if(a==1){
			int b;
			cin>>b;
			push(b);
		}
		if(a==2)
			pop();
		if(a==3)
			out();
		if(a==4){
			int b;
			cin>>b;
			open(b);
		}
		if(a==5)
			save();
	}
	return 0;
}