看起来似乎是被 $n = 2 \times 10^5, x = 1$ 的数据卡了...

但是问题出在哪里呢 /dk

#include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <unordered_map>
#define ai_jiang signed main()
#define rep(a, b, c) for (int a = b; a <= c; a++)
#define drep(a, b, c) for (int a = b; a >= c; a--)
#define all(a) a.begin(), a.end()
using namespace std;
using ll = long long;
using poly = vector<ll>;
namespace ai
{
    constexpr int N = 2e5 + 5, M = 1e9;
    ll t, n, x, a[N << 1];
    unordered_map<ll, ll> xx;
    set<ll> v;
}
using namespace ai;
ai_jiang
{
    cin.tie(nullptr);
    cout.tie(nullptr);
    ios::sync_with_stdio(0);
    cin >> t;
    while (t--)
    {
        cin >> n >> x;
        xx.clear();
        v.clear();
        rep(i, 1, n) cin >> a[i], xx[a[i]]++, v.insert(a[i]);
        for (auto i : v)
        {
            if (xx[i] > 1)
            {
                if (!xx[i + x])
                    v.insert(i + x); 
                xx[i + x] += xx[i] - 1, xx[i] = 1;
            }
        }
        if (*v.begin() != 0)
            cout << 0 << '\n';
        else
        {
            auto it = v.begin();
            int f = 0;
            rep(i, 0, (int)v.size() - 2)
            {
                auto k = it;
                ++k;
                int val = *it, vl = *k;
                if (val + 1 < vl)
                {
                    cout << val + 1 << '\n';
                    f = 1;
                    break;
                }
                it++;
            }
            if(!f) cout<<(*(--v.end()))+1<<'\n';
        }
    }
    return 0;
}