The first repeat
A stream of ids is meant to be unique; the audit flags the first value that turns up twice.
- Scan left to right and stop at the first value that has appeared earlier.
- A value repeated later does not matter if a smaller repeat came first.
- No value repeats at all gives -1.
FirstRepeating(values: 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 FirstRepeating(List<int> values) {
}Worked examples
| Call | Result |
|---|---|
FirstRepeating(new List<int> { 2, 1, 3, 1 }) | 1 |
FirstRepeating(new List<int> { 1, 2, 3 }) | -1 |
FirstRepeating(new List<int> { 4, 4 }) | 4 |
FirstRepeating(new List<int> { 7, 7, 7 }) | 7 |
Hint
A set of everything so far; the moment you add one that is already there, that is the answer.
Reference solution in C#
public int FirstRepeating(List<int> values) {
var seen = new HashSet<int>();
foreach (var v in values) {
if (seen.Contains(v)) return v;
seen.Add(v);
}
return -1;
}