阿牛的EOF牛肉串-记忆化搜索或动态规划

/*
Author: 2486
Memory: 1416 KB		Time: 0 MS
Language: G++		Result: Accepted
*/
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int maxn=40+5;
int n;
long long  dp[maxn][3];//记录某个位置选择了什么
long long  dfs(int pos,int preindex){//能够直接用dp取代。
    if(pos==0)return 1;
    if(preindex!=-1&&dp[pos+1][preindex]!=-1)return dp[pos+1][preindex];
    long long  ans=0;
    for(int i=0;i<3;i++){
        if(preindex==1&&i==1)continue;
        ans+=dfs(pos-1,i);
    }
    if(preindex!=-1)
    dp[pos+1][preindex]=ans;
    return ans;
}
int main() {
    memset(dp,-1,sizeof(dp));
    while(~scanf("%d",&n)) {
            printf("%lld\n",dfs(n,-1));
    }
    return 0;
}