请问这道题怎么用深搜做

#include<iostream>
using namespace std;
int n,book[20],ans;
void dfs(int xh,int top)
{
	top++; 
	if(xh==n)
	{
		ans++;
		return ;	
	}
	if(xh<n)
	{
		dfs(xh+1,top);
		dfs(xh+1,top-1);		
	}	
	return ;	
}
int main()
{
	cin>>n;
	dfs(1,0);
	cout<<ans;
	return 0;
}