python/TangDou/LuoGuBook/P2638.cpp

#include <bits/stdc++.h>

//https://www.luogu.com.cn/blog/x4Cx58x54/solution-p2638
//经典题

using namespace std;
typedef unsigned long long ULL;
int n, a, B;
ULL ans;

//求组合数的优化后办法
//优点：从1开始除和乘，可以防止过早溢出和除法除不尽
ULL C(int n, int m) {
    ULL sum = 1;
    for (int i = 1; i <= m; i++)
        sum = sum * (n - m + i) / i;
    return sum;
}

int main() {
    cin >> n >> a >> b;
    for (int i = 0; i <= a; i++)
        for (int j = 0; j <= b; j++)
            ans += C(n + i - 1, n - 1) * C(n + j - 1, n - 1);

    cout << ans << endl;
    return 0;
}