Implement common data-structure and JS tasks

You are given several independent coding tasks that appeared across an OA and frontend-focused interviews. Implement each as specified. ## 1) Time-based key-value store Design a data structure that supports storing multiple values for the same key at different timestamps. ### Operations - `set(key: string, value: string, timestamp: int) -> void` - `get(key: string, timestamp: int) -> string` ### `get` rule Return the value associated with `key` at the **largest** timestamp `t` such that `t <= timestamp`. If no such timestamp exists for that key, return an empty string. ### Constraints (typical) - Many calls; aim for ~`O(log n)` per `get` per key. - Timestamps for a given key are non-decreasing across `set` calls. --- ## 2) Remove duplicates from a linked list Given the head of a **sorted** singly linked list, remove duplicates so that each value appears only once. - Input: `head` of a sorted list. - Output: the modified list head. Example: `1 -> 1 -> 2 -> 3 -> 3` becomes `1 -> 2 -> 3`. --- ## 3) Implement a curried “add” function in JavaScript Implement a JavaScript function `add` that supports chained calls via currying. ### Required behavior (pick one clear contract and implement it) Option A (explicit terminator): - `add(1)(2)(3)()` returns `6`. Option B (implicit coercion): - `add(1)(2)(3) == 6` is `true` (via `valueOf`/`toString`). Handle common edge cases (e.g., `add(0)`, negative numbers). --- ## 4) Traffic light scheduler (frontend async) Implement a function that repeatedly cycles through traffic lights in order: - Red for 3 seconds - Green for 2 seconds - Yellow for 1 second Requirements: - The sequence must repeat indefinitely (or for `n` cycles if you choose to add a parameter). - Use asynchronous control flow (Promises / `async`/`await` / timers). - Each transition should trigger a callback/log such as `console.log('red')` at the moment the light turns on.