luogu P4781 【模板】拉格朗日插

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
typedef long long ll;
const ll mod = 998244353;
ll xx[maxn], yy[maxn];
ll qpow(ll a, ll b) {
    ll ans = 1;
    while (b) {
        if (b & 1) {
            ans = (ans * a) % mod;
        }
        b >>= 1;
        a = (a * a) % mod;
    }
    return ans % mod;
}
int main() {
    ll n, k;
    cin >> n >> k;
    for (int i = 1; i <= n; i++) {
        cin >> xx[i] >> yy[i];
    }
    ll ans = 0;
    for (int i = 1; i <= n; i++) {
        ll ret = 1, res = 1;
        for (int j = 1; j <= n; j++) {
            if (i == j)continue;
            ret *= (k - xx[j]);
            res *= (xx[i] - xx[j]);
            ret = (ret % mod + mod) % mod; 
            res = (res % mod + mod) % mod;
        }
        ll tmp = qpow(res, mod - 2);
        ans += yy[i] * tmp % mod * ret % mod;
        ans %= mod;
    }
    cout << ans << endl;
    return 0;
}