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 + ;
const int M =  + ;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + ;
const double eps = 1e-;

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

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

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