## Problem Restaurant IDs arrive one at a time. Support two operations: - `add(value)`: append one occurrence. - `showFirstUnique()`: return the earliest-arriving value that has appeared exactly once so far, or `-1` if no such value exists. Process an operation sequence and return the result of every `show` operation. ### Function Contract Implement `firstUniqueStream(operations)`, where operations are `['add', value]` or `['show']`. Return an integer array. ### Constraints & Assumptions - `0 <= len(operations) <= 200,000`. - Values are signed 64-bit integers; `-1` is reserved for the no-result output and will not be added. - Once a value appears a second time, later occurrences can never make it unique again. - Operations are processed in the supplied order. ### Clarifying Questions to Ask - Does “first” mean smallest value? No, earliest arrival among currently unique values. - Can a value become unique again? No; counts only increase. - What is returned before anything is added? `-1`. - Must `show` mutate the state? No. ```hint Keep only current candidates in arrival order Use a count map plus an ordered structure containing values whose count is exactly one. Remove a value when its count reaches two. ``` ```hint Lazy deletion is also valid A queue may keep stale repeated values if each `show` removes stale elements from its front using the count map. ``` ### Example ```text operations = [ ["add", 4], ["add", 7], ["show"], ["add", 4], ["show"], ["add", 7], ["show"] ] ``` Return `[4, 7, -1]`. ### Evaluation Focus - Preserves arrival order among values that remain unique. - Removes or skips a value exactly when its second occurrence arrives. - Supports repeated `show` calls without changing the answer. - Runs in amortized `O(1)` time per operation with `O(u)` state for `u` distinct values. ### Extensions to Discuss 1. How could old values be evicted in a fixed-time window? 2. What memory remains necessary to distinguish a second from a third occurrence? 3. How would concurrent producers change ordering and synchronization?