bzoj 1224

dfs + 剪枝， 用最大最小值剪。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define pii pair<int,int>
#define piii pair<int, pair<int,int> >

using namespace std;

const int N = 5e4 + 7;
const int M = 10 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-10;

int m, n, x, y, ans;
double sum[55];
double w;

void dfs(int x, int y, double p) {
    if(p + sum[x + (n - y) - 1] - sum[x - 1] - w < -eps) return;
    if(p + sum[m] - sum[m - (n - y)] - w > eps) return;
    if(y == n) {
        ans++;
        return;
    }
    dfs(x + 1, y, p);
    dfs(x + 1, y + 1, p + 1.0 / x);
}

int main() {
    scanf("%d%d%d%d", &n, &m, &x, &y);
    w = 1.0 * x / y;
    for(int i = 1; i <= m; i++) {
        sum[i] = sum[i - 1] + 1.0 / i;
    }
    dfs(1, 0, 0.0);
    printf("%d
", ans);
    return 0;
}
/*
*/