题解 | 前缀平方和序列喵

注意到答案为 喵~

#include <cmath>
#include <iostream>
using namespace std;

using ll = long long;

const int MOD = 1e9 + 7;

ll Pow(ll a, int b) {
    ll res = 1;
    while (b) {
        if (b & 1) {
            res = res * a % MOD;
        }
        a = a * a % MOD;
        b >>= 1;
    }
    return res;
}

ll Inv(ll a) {
    return Pow(a, MOD - 2);
}

int n;
int x;
int cs;

void Solve() {
    cin >> n >> x;
    cs = sqrt(x + 0.5);
    if (n > cs) {
        cout << '0';
        return;
    }
    ll zi = 1;
    ll mu = 1;
    for (int i = cs; i > cs - n; i--) {
        zi = zi * i % MOD;
    }
    for (int i = n; i > 1; i--) {
        mu = mu * i % MOD;
    }
    cout << zi* Inv(mu) % MOD;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    Solve();
}
// 64 位输出请用 printf("%lld")