delete pics to save space

2.1639.cpp

@@ -0,0 +1,35 @@
+#include<cstdio>
+#include<cmath>
+//C(n,m)=n!/(m!(n-m)!)
+const int maxn=200004;
+long double logF[maxn*2+1];
+inline long double logC(int n,int m)
+{
+	return logF[n]-logF[m]-logF[n-m];
+}
+double solve(int n,double p)
+{
+	double res=0;
+	for(int i=0;i<=n;i++)
+	{
+		long double c=logC(n*2-i,n);
+		long double v1=c+(n+1)*log(p)+(n-i)*log(1-p);
+		long double v2=c+(n+1)*log(1-p)+(n-i)*log(p);
+		res+=i*(exp(v1)+exp(v2));
+	}
+	return res;
+}
+int kase=0,n;
+double p;
+int main()
+{
+#ifdef local
+	freopen("pro.in","r",stdin);
+#endif 
+	logF[0]=0;
+	for(int i=1;i<=maxn;i++)
+		logF[i]=logF[i-1]+log(i);
+	while(scanf("%d%lf",&n,&p)==2)
+		printf("Case %d: %.6lf\n", ++kase, solve(n, p));
+	return 0;
+}