loj #6191. 「美团 CodeM 复赛」配对游戏 期望dp

#include <bits/stdc++.h>    
#define N 2004   
#define LL long long 
#define setIO(s) freopen(s".in","r",stdin) 
using namespace std;    
double p[N][N],f[N][N];   
int main() 
{ 
	// setIO("input"); 
	int i,j,n; 
	scanf("%d",&n), p[0][0]=1;   
	double ans=0.0; 
	for(i=0;i<n;++i)        
	{
		p[i+1][1]+=p[i][0]/2, f[i+1][1]+=(f[i][0]+p[i][0])/2;                    
		p[i+1][0]+=p[i][0]/2, f[i+1][0]+=(f[i][0]+p[i][0])/2;              
		for(j=1;j<n;++j) 
		{
			p[i+1][j+1]+=p[i][j]/2, f[i+1][j+1]+=(f[i][j]+p[i][j])/2;             
			p[i+1][j-1]+=p[i][j]/2, f[i+1][j-1]+=(f[i][j]-p[i][j])/2;       
		}
	} 
	for(i=0;i<=n;++i)   ans+=f[n][i];    
	printf("%.3f\n",ans);         
	return 0; 
}