Problem
Given an array, find the k-th element that becomes a duplicate — the k-th distinct value (in scan order) to reach a second occurrence.
Input / Output
- Input: integer array, integer k.
- Output: the k-th value to hit its second occurrence, or a sentinel if fewer than k such values exist.
Constraints
- 1 ≤ n ≤ 10^5.
- Ordering is by when each value first reaches count 2, not by the value itself.
Example
- [1,2,1,3,2,1], k=2 -> 2 (value 1 reaches count 2 first, then value 2)