HDU 5819 Knights

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
const int MAXN=1005;
const int inv_2=500000004;
const int mod=1000000007;
inline int read(){
	int x=0,f=1,ch=getchar();
	while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=-1;ch=getchar();}
	while(ch>=‘0‘&&ch<=‘9‘){x=x*10+ch-‘0‘;ch=getchar();}
	return x*f;
}
inline int quick_pow(int x,int p){
	if(!p) return 1;
	int res=1;
	if(p&1) res=x;
	p>>=1;
	int kk=quick_pow(x,p);
	return 1ll*res*kk%mod*kk%mod;
}
int a[MAXN];
int dp[MAXN][MAXN];
int main(){
	int T=read();
	for(int cas=1;cas<=T;cas++){
		int n=read();
		for(int i=1;i<=n;i++) a[i]=read();
		dp[1][1]=1;
		for(int i=2;i<n;i++){
			if(a[i]){
				for(int j=i;j>1;j--) dp[i][j]=dp[i-1][j-1];
				dp[i][1]=0;
			}
			else{
				dp[i][i]=0;
				for(int j=i-1;j>1;j--) dp[i][j]=(1ll*dp[i-1][j]*inv_2%mod+1ll*dp[i][j+1]*inv_2%mod)%mod;
				dp[i][1]=(dp[i-1][1]+dp[i][2])%mod;
			}
		}
		int ans=0,kk=inv_2;
		for(int i=1;i<n;i++) ans=(ans+1ll*dp[n-1][i]*kk%mod)%mod,kk=1ll*kk*inv_2%mod;
		printf("Case #%d: %d\n",cas,ans);
	}
	return 0;
}