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
Go 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
func firstMissingSeq(sequence []int) int {
}Worked examples
| Call | Result |
|---|---|
firstMissingSeq([]int{1, 2, 3}) | 0 |
firstMissingSeq([]int{0, 1, 3}) | 2 |
firstMissingSeq([]int{0, 1, 2}) | 3 |
firstMissingSeq([]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 Go
func firstMissingSeq(sequence []int) int {
length := len(sequence)
present := make([]bool, length+1)
for _, v := range sequence {
if v >= 0 && v <= length {
present[v] = true
}
}
for i := 0; i <= length; i++ {
if !present[i] {
return i
}
}
return length + 1
}