CodeForces 148D: Bag of mice

#include <cstdio>
#include <cstdlib>

double p[1004][1004];

int main() {
    int w, b;
    while (scanf("%d%d", &w, &b) != EOF) {

        //其实可以放到外面只初始化一次
        for (int i = 0; i <= b; i++) {
            p[0][i] = 0;
        }

        for (int i = 1; i <= w; i++) {
            for (int j = 0; j <= b; j++) {
                double sum = i+j+0.0;
                p[i][j] = i/sum;
                if (i+j >= 3 && j >= 2) {
                    double remain = 0;
                    if (j >= 3) {
                        remain += ((j-2+0.0)/(sum-2))*p[i][j-3];
                    }
                    remain += ((i+0.0)/(sum-2))*p[i-1][j-2];
                    p[i][j] += ((j+0.0)/sum) * ((j-1+0.0)/(sum-1.0)) * remain;
                }
            }
        }
        printf("%.10lf\n", p[w][b]);
    }
    return 0;
}