Dpuvalive 6177the King's ups and downs

I wrote an explosive search during the exercise competition, and then I won't do it if I didn't run it out. I said I ran it with a shape and played the table. In fact, there is a direct DP. The following is the DP of the table .. The result is multiplied by 2 except the first one.

#include <cstdio>#include <cstring>#include <algorithm>#include <climits>#include <string>#include <iostream>#include <map>#include <cstdlib>#include <list>#include <set>#include <queue>#include <stack>#include<math.h>using namespace std;typedef long long LL;LL n;LL dp[1<<20][20][2];LL dfs(LL state,LL pre,LL shang){    if(~dp[state][pre][shang]) return dp[state][pre][shang];    LL x=0;    for(LL i=0;i<n;i++)        if(state&(1<<i)) x++;    if(x==n) return dp[state][pre][shang] = 1;    LL ans=0;    if(shang){        for(LL i=pre+1;i<=n;i++)        if(state&(1<<(i-1))) continue;        else        ans+=dfs(state|(1<<(i-1)),i,shang^1);    }    if(!shang){        for(LL i=1;i<pre;i++){            if(state&(1<<(i-1))) continue;            ans+=dfs(state|(1<<(i-1)),i,shang^1);        }    }    return dp[state][pre][shang]= ans;}int main(){    freopen("out.txt","w",stdout);    for(n=1;n<=20;n++){        memset(dp,-1,sizeof(dp));        cout<<dfs(0,0,1)<<‘,‘<<endl;    }    return 0;}