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概率 \(dp\) + 双指针

占个坑，说明补了这题，题解没时间写了，先咕咕了

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const double eps = 1e-8;

int dcmp(double x)
{
    return (x > eps) - (x < -eps);
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    int n, q, k;
    cin >> n >> k >> q;
    vector<ll>a(n + 1), sum(n + 1, 0);
    for(int i=1; i<=n; i++) cin >> a[i];
    vector<ll> b = a;
    sort(a.begin() + 1, a.end());
    for(int i=1; i<=n; i++) sum[i] = sum[i - 1] + a[i];
    vector<double>E(k + 1);
    for(int i=1; i<=n; i++) E[0] += (n - 1) * a[i];
    E[0] /= 1ll * n * (n - 1) / 2;
    for(int i=1; i<=k; i++)
    {
        double p = E[0], tot = 0;
        int l = 1, r = n;
        ll cnt = 0;
        while(l <= n)
        {
            while(r >= l && a[r] + a[l] >= E[i - 1]) r--;
            if(l > r) break;
            ll len = r - l;
            p -= (len * a[l] + sum[r] - sum[l]) / (1.0 * n * (n - 1)) * 2;
            tot += len;
            l++;
        }
        p += E[i - 1] / (1ll * n * (n - 1)) * 2 * tot;
        E[i] = p;
    }
    cout << fixed << setprecision(20) << E[k] << "\n";
    while(q--)
    {
        ll x, y, c;
        cin >> x >> y >> c;
        if(c == 0) cout << "accept\n";
        else
        {
            int dif = dcmp(b[x] + b[y] - E[c - 1]);
            if(dif == 0) cout << "both\n";
            else if(dif == 1) cout << "accept\n";
            else cout << "reselect\n";
        }
    }
    return 0;
}