//InputSequence contains only unique numbers
//each sequence has two entries in seq: (left boundary => right boundary) + (right boundary => left boundary)

Dictionary<int, int> seq = new Dictionary<int, int>()
foreach(int n in InputSequence)
{
    seq[n] = n;
    var lb = n,rb = n;
    if (seq.contains(n - 1))
    {
        lb = seq[n - 1]
        seq.remove(n - 1)
        seq[lb] = rb;
        seq[rb] = lb;
    }
    if (seq.contains(n + 1))
    {
        rb = seq[n + 1];
        seq.remove(n + 1);
        seq[lb] = rb;
        seq[rb] = lb;
    }
}