Design a Browser Tab Handler

Problem Statement

Design a Tab Handler (like a browser's tab manager) that supports:

1. openTab(url) — Open a new tab with the given URL, assign a unique ID
2. openTab(url, force) — Open a tab even if another tab already has the same URL
3. closeCurrentTab() — Close the currently active tab
4. closeTab(id) — Close a tab by its unique ID
5. getCurrentTab() — Return the currently active tab
6. listTabs() — List all currently open tabs in order

Aim for efficient operations (O(1) where possible).

Constraints

• Each tab has a unique auto-incrementing ID
• Tabs maintain their open order
• Closing the current tab should activate an adjacent tab
• At most 10^5 operations

Example

openTab("google.com")     → id=1, current=1
openTab("github.com")     → id=2, current=2
openTab("google.com")     → duplicate URL, no new tab (unless forced)
openTab("google.com", true) → id=3, current=3 (forced)
getCurrentTab()           → tab 3
closeTab(2)               → removes github tab
listTabs()                → [google(1), google(3)]

What the Interviewer Expects

1. HashMap + Doubly Linked List — the ideal combo:
   • HashMap<int, Node>: ID → tab node for O(1) lookup by ID
   • Doubly Linked List: maintains tab order + current pointer, O(1) insert/delete
2. Node structure — each node holds id, url, prev, next.
3. Duplicate URL handling — without force, check if URL already open (may need a URL→node index). With force, always create new.
4. Current tab pointer — track which node is active. On close, move to prev or next.
5. This combines data-structure selection with API design — not a pure algorithm problem. Show clean interface design.

Follow-ups

1. How would you add "reopen last closed tab" (undo)?
2. How would you implement tab history (back/forward within a tab)?
3. How would you handle tab groups or pinned tabs?
4. What if you needed to find a tab by URL in O(1)? What structure would you add?