递推关系123不满足归路 也要赋值给3

include <bits/stdc++.h>

using namespace std;
long long dp[55];
long long f(int x)//注意数据量会很大防爆!
{
if(dp[x]!=-1) return dp[x];
return dp[x]=f(x-1)+f(x-2)*2;//别和数学的弄混了
}
int main()
{
int n;
while(cin>>n)
{
memset(dp,-1,sizeof(dp));
dp[0]=0;dp[1]=3;dp[2]=6;dp[3]=6;
cout<<f(n)<<endl;
}
}
应用公式的话

include <stdio.h>

include <math.h>

int main()
{
int N;
while(scanf("%d",&N)1)
{
long long result;
if(N1) result=3;
else result=pow(2,N)+pow(-1,N)*2;
printf("%lld\n",result);
}
}