zoj 3582 Back to the Past (概率dp)

#include<iostream>
#include<math.h>
#include<stdio.h>
#include<algorithm>
#include<string.h>
#include<string>
#include<vector>
#include<queue>
#include<map>
#include<set>
#include<stack>
#define B(x) (1<<(x))
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef unsigned ui;
const int oo = 0x3f3f3f3f;
//const ll OO = 0x3f3f3f3f3f3f3f3f;
const double eps = 1e-9;
#define lson rt<<1
#define rson rt<<1|1
void cmax(int& a, int b){ if (b > a)a = b; }
void cmin(int& a, int b){ if (b < a)a = b; }
void cmax(ll& a, ll b){ if (b > a)a = b; }
void cmin(ll& a, ll b){ if (b < a)a = b; }
void cmax(double& a, double b){ if (a - b < eps) a = b; }
void cmin(double& a, double b){ if (b - a < eps) a = b; }
void add(int& a, int b, int mod){ a = (a + b) % mod; }
void add(ll& a, ll b, ll mod){ a = (a + b) % mod; }
const ll MOD = 1000000007;
const int maxn = 55;
double dp[maxn][maxn];
double C[maxn][maxn];
double p1[maxn], p2[maxn];

void GetC(){
	for (int i = 0; i < maxn; i++){
		C[i][0] = 1.0;
		for (int j = 1; j < i; j++)
			C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
		C[i][i] = 1.0;
	}
}

void Init(int n, double p){
	p1[0] = 1.0; p2[0] = 1.0;
	for (int i = 1; i <= n; i++){
		p1[i] = p1[i - 1] * p;
		p2[i] = p2[i - 1] * (1.0 - p);
	}
}

int main(){
	int n, m;
	double p;
	GetC();
	while (scanf("%d %d %lf", &n, &m, &p) != EOF){
		if (n == 0 && m == 0)
			break;
		Init(n, p);
		memset(dp, 0, sizeof dp);
		for (int i = n; i >= 0; i--)
		for (int j = n; j >= 0; j--){
			if (i >= m  && j >= m)
				continue;
			for (int x = 0; x + i <= n; x++)
			for (int y = 0; y + j <= n; y++){
				if (x == 0 && y == 0)
					continue;
				if (x <= n - i&&y <= n - j)
					dp[i][j] += dp[i + x][j + y] * C[n - i][x] * p1[x] * p2[n - x - i] * C[n - j][y] * p1[y] * p2[n - y - j];
			}
			dp[i][j] = (dp[i][j] + 1.0) / (1.0 - p2[n - i] * p2[n - j]);
		}
		printf("%.6lf\n", dp[0][0]);
	}
	return 0;
}