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Problems

Basics first, then the classics, then company-style assessments. Every problem has tests you run right here; multi-level ones unlock as you go. See the roadmap.

Linked lists

LRU / LFU cache

Hash map + doubly linked list for O(1) get/put.

Visual guide Quiz (10) Spot the pattern
Notes

Recognise it when: "O(1) get/put with eviction", caching, most-recently-used ordering.

A dict maps keys to nodes, plus a doubly linked list with sentinel head and tail: head.next is the most recent, tail.prev the least recent.

  • get: unlink the node, push it to the front.
  • put: update and move it, or insert at the front; if over capacity, unlink tail.prev and delete its key from the dict.

Gotchas

  • Store the key in the node so eviction can delete it from the dict.
  • Updating an existing key refreshes its recency.
  • OrderedDict.move_to_end / popitem(last=False) is fine in real code, but interviewers usually want the list.
  • LFU: a dict of frequency -> OrderedDict, plus a min_freq tracker.

6 problems

Interview roadmap

Linked lists Reverse, merge, detect cycles, build an LRU cache.

LRU / LFU cache guide

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