Second-Smallest Unique Element

Return the second-smallest value among elements that occur exactly once in the input array. Intern-level Superday warm-up. Requirements Input: an integer array (may contain duplicates). Output: the second-smallest value among elements that appear exactly once in the input. If fewer than two unique values exist, behavior was not pinned down — clarify with the interviewer (typical answers: throw, return null, return -1). Notes Two-pass solution: build a Map<value, count>; then scan the values where count == 1 and track the two smallest. O(n) time, O(n) space. One-pass alternative: maintain smallest and secondSmallest slots, plus a Set of values seen more than once that must be excluded; trickier to implement correctly. Edge cases: input length < 2; all elements duplicates; negative values (don't assume positive). Preparation Implement the two-pass form (clean and explicit); then mention the one-pass form as a follow-up. Drill the related LC 215 "Kth Largest Element" for muscle memory on quickselect, which Goldman interviewers occasionally offer as a generalization.