Google CodeJam浴室档位2017年资格赛圆形大数据集错误

Question

我正在尝试解决Google CodeJam 2017 "Bathroom Stalls" problem C - 解决方案在链接中提供，我的C＃代码在small1和2集上运行良好。大集看起来没问题，但在线评判认为它不正确。有谁有想法为什么？我尝试使用ulongs，我试图检测溢出，我通过bruteforce与小集进行比较以找到相同的解决方案。

    private string SeatTreeSkip(ulong n, ulong k)
    {
        var q = new SortedSet<ulong>();
        q.Add(n);
        var c = new Dictionary<ulong,ulong>();
        c[n] = 1;
        for (ulong i = 0; i < n; )
        {
            ulong p = q.Last();
            ulong x0 = (ulong)Math.Ceiling((p - 1) / 2.0);
            ulong x1 = (ulong)Math.Floor((p - 1) / 2.0);
            if (p - 1 < 0)
                p = p;
            i += c[p];

            if (i < 0 || x0 < 0 || x1 < 0)
                throw new Exception("overflow");
            if (i >= k)
                return x0 + " " + x1;

            q.Remove(p);
            q.Add(x0);
            q.Add(x1);
            if (!c.ContainsKey(x0)) c.Add(x0, 0);
            if (!c.ContainsKey(x1)) c.Add(x1, 0);
            c[x0] += c[p];
            c[x1] += c[p];
        }
        throw new Exception("k is over");
    }

输入代码段：

4 2
5 2
6 2
1000 1000
1000 1
500000000000000000 249999999999999999
1000000000000000000 500000000000000000
999999999999999999 423539247696576511
3 2
500000000000000000 144115188075855872
1000000000000000000 1

输出片段：

Case #1: 1 0
Case #2: 1 0
Case #3: 1 1
Case #4: 0 0
Case #5: 500 499
Case #6: 1 0
Case #7: 1 0
Case #8: 1 1
Case #9: 0 0
Case #10: 1 1
Case #11: 500000000000000000 500000000000000000