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
Java 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
int firstRepeating(List<Integer> values) {
}Worked examples
| Call | Result |
|---|---|
firstRepeating(Main.<Integer>ls(2, 1, 3, 1)) | 1 |
firstRepeating(Main.<Integer>ls(1, 2, 3)) | -1 |
firstRepeating(Main.<Integer>ls(4, 4)) | 4 |
firstRepeating(Main.<Integer>ls(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 Java
int firstRepeating(List<Integer> values) {
Set<Integer> seen = new HashSet<>();
for (int v : values) {
if (seen.contains(v)) return v;
seen.add(v);
}
return -1;
}