The first missing sequence number
A reliable-transport layer assigns monotonically increasing sequence numbers, but packets can arrive out of order or be lost. Find the lowest number starting from zero that has not been seen.
- Return the smallest non-negative integer that is absent from the sequence.
- The sequence may contain duplicates.
- An empty sequence means zero has never arrived: return 0.
FirstMissingSeq(sequence: list<int>) → int
C# needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
public int FirstMissingSeq(List<int> sequence) {
}Worked examples
| Call | Result |
|---|---|
FirstMissingSeq(new List<int> { 1, 2, 3 }) | 0 |
FirstMissingSeq(new List<int> { 0, 1, 3 }) | 2 |
FirstMissingSeq(new List<int> { 0, 1, 2 }) | 3 |
FirstMissingSeq(new List<int> { }) | 0 |
Hint
Build a boolean presence array covering 0 through the list length, mark every value that falls in range, then scan for the first gap.
Reference solution in C#
public int FirstMissingSeq(List<int> sequence) {
int length = sequence.Count;
bool[] present = new bool[length + 1];
foreach (int v in sequence) {
if (v >= 0 && v <= length) present[v] = true;
}
for (int i = 0; i <= length; i++) {
if (!present[i]) return i;
}
return length + 1;
}