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.