C++(clang++ 11.0.1) 解法, 执行用时: 415ms, 内存消耗: 444K, 提交时间: 2022-10-17 02:08:39

#include <iostream>
using namespace std;
int n, m, x, y;
int ans = 0;
double sum[105];
double obj;
const double eps = 1e-12;

void dfs(double x, int cur = 1, int f = 0) {
    if (f == n) {
        if (x >= -eps && x <= eps) {
            ans++;
        }
        return;
    }
    if (cur > m) return;
    if (x - eps > sum[min(cur + n - f - 1, m)] - sum[cur - 1]) return;
    if (x + eps < sum[m] - sum[m - (n - f)]) return;
    dfs(x, cur + 1, f);
    if (x + eps >= 1.0 / cur) {
        dfs(x - 1.0 / cur, cur + 1, f + 1);
    }
}

int main() {
    cin >> n >> m >> x >> y;
    for (int i = 1; i <= m; i++) sum[i] = sum[i - 1] + 1.0 / i;
    dfs((double) x / y);
    cout << ans << endl;
    return 0;
}