题解

#include<bits/stdc++.h>
using namespace std;
const int maxn = 20;
int n;
long long f[maxn][maxn];
long long dfs(int i,int j)
{
	if(f[i][j]) return f[i][j]; 
	if(i==0)return 1;
	if(j>0) f[i][j]+=dfs(i,j-1);
	f[i][j]+=dfs(i-1,j+1);
	return f[i][j];
}
int main()
{
	cin>>n;
	cout<<dfs(n,0);
	return 0;
}

直接dfs