要用之前的树作为下一轮的子树，而不是在之前的树的基础上加新的节点

#include<iostream>
#define MAX 21
using namespace std;
long long dp[MAX]={0};

int main(){
    int n;
    cin >> n;
    dp[0] = (long long)1;
    dp[1] = (long long)1;
    dp[2] = (long long)2;
    for(int i = 3; i <= n; i++){
        for(int j = 0; j < n; j++ ){
            dp[i] += (long long)dp[i-1-j]*dp[j];
        }

    }
    cout << dp[n] << endl;
    return 0;
}