Problem
In an Alten Nederland mobile app telemetry pipeline, event IDs should be unique within a single upload batch. Given a list of integers event_ids, return the first value that appears more than once when scanning from left to right. If no duplicate exists, return -1.
Formal Specification
- Input:
event_ids, a list of integers - Output: An integer representing the first duplicated value encountered during a left-to-right scan, or
-1if all values are unique
A value is considered the first duplicate if its second occurrence appears before the second occurrence of any other duplicated value.
Constraints
- 1 <= len(event_ids) <= 10^5
- -10^9 <= event_ids[i] <= 10^9
- Return the value whose second occurrence appears first
- If no duplicate exists, return -1
Function Signature
def first_duplicate(event_ids):
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