Input: a list of integers, with repeated entries allowed.
Output: return the second-lowest number among values whose frequency in the list is exactly one.
When fewer than two values occur only once, the expected result is unspecified and should be discussed with the interviewer (common choices include throwing an error, returning null, or returning -1).
Notes
Watch for short inputs with fewer than two elements, arrays made entirely of duplicates, and negative integers; do not presume every value is positive.
Preparation
For additional practice, review LC 215, "Kth Largest Element," to refresh quickselect; interviewers may sometimes present it as a broader variant.