ZOJ 10th Anniversary Contest - B Back to the Past

#include<stdio.h>
#include<string.h>
inline void swap(int & a,int & b)
{
    int k=a;
    a=b;
    b=k;
}
const int N=50;
long long binomial[N+1][N+1];
void init()
{
    int i,j;
    for (i=0;i<=N;i++)
    {
        binomial[i][0]=binomial[i][i]=1;
        for (j=1;j<i;j++)
        {
            binomial[i][j]=binomial[i-1][j-1]+binomial[i-1][j];
        }
    }
}
double p1[N+1]={1},p0[N+1]={1};
inline double oneDay(int n,int m)
{
    return binomial[n][m]*p1[m]*p0[n-m];
}
double f[N+1][N+1][N+1][N+1];
bool calc[N+1][N+1][N+1][N+1];
double getF(int n1,int m1,int n2,int m2)
{
    if (m1<0) m1=0;
    if (m2<0) m2=0;
    if (m1==0&&m2==0) return 0;
    if (n1<n2)
    {
        swap(n1,n2);
        swap(m1,m2);
    }
    double & ff=f[n1][m1][n2][m2];
    if (!calc[n1][m1][n2][m2])
    {
        ff=oneDay(n1,0)*oneDay(n2,0);
        int k1,k2;
        for (k1=0;k1<=n1;k1++) for (k2=0;k2<=n2;k2++)
        {
            if (k1==0&&k2==0) continue;
            double p=oneDay(n1,k1)*oneDay(n2,k2);
            ff+=p*(1+getF(n1-k1,m1-k1,n2-k2,m2-k2));
        }
        ff/=(1-oneDay(n1,0)*oneDay(n2,0));
        calc[n1][m1][n2][m2]=true;
    }
    return ff;
}
int main()
{
    init();
    int n,m;
    double p;
    while (1)
    {
        scanf("%d%d%lf",&n,&m,&p);
        if (n==0&&m==0&&p==0)
        {
            break;
        }
        int i;
        for (i=1;i<=n;i++)
        {
            p1[i]=p1[i-1]*p;
            p0[i]=p0[i-1]*(1-p);
        }
        memset(calc,false,sizeof(calc));
        printf("%.6f\n",(double)getF(n,m,n,m));
    }
    return 0;
}