Visualized DSA Strategies

Episode 2 Hashing: HashSet and HashMap

1 h 2 min · Python · Quiz

Title card of Episode 2: Hashing: HashSet and HashMap

▶ This video is coming to YouTube soon.

The complete guide to hashing for coding interviews, in Python. First how a hash set really works: buckets, collisions and why lookups are O(1) on average; the four questions hashing answers; what each structure costs; sets and dicts in Python (ask with in, because add returns None; get with a default; Counter and defaultdict); and five traps. Then nine types, each worked as a small problem in full: examples, the brute force animated beside the running line of code, the complete code, every line explained, and the hashing solution the same way.

What you'll learn

  • How a hash function, buckets and collisions make lookups O(1) on average
  • Sets and dicts in Python: add, in, discard, get with a default, setdefault, Counter, defaultdict and items
  • Seen before? A set, and a set as a window of the last k numbers
  • Look up the complement: Two Sum with a dict
  • Counting with a dict or a Counter, and with a list of 26 counts
  • Grouping by a key with defaultdict(list), and set operations on two lists
  • The longest consecutive run: only start where a run starts
  • Designing a set with a random pick: a dict plus a list
  • Five traps: is vs ==, KeyError on a missing key, lists as keys, __eq__ without __hash__, changing a dict while looping over it

Chapters

  1. 0:00Intro
  2. 0:22How a hash set works
  3. 1:34Four questions hashing answers
  4. 2:01What each structure costs
  5. 2:42Sets and dicts in Python
  6. 4:24Five traps
  7. 5:30Nine hashing types
  8. 6:10Type 1 · Is there a duplicate?
  9. 11:52Type 2 · Duplicates close together
  10. 17:53Type 3 · Two numbers that add up
  11. 23:36Type 4 · Are they anagrams?
  12. 29:06Type 5 · Can the note be written?
  13. 35:11Type 6 · Group the anagrams
  14. 40:50Type 7 · What do two arrays share?
  15. 46:51Type 8 · Longest consecutive run
  16. 53:03Type 9 · A set with a random pick
  17. 59:44Spot the type
  18. 1:00:31Quiz: which type?
  19. 1:01:18Recap

Quiz: test yourself

The 5 questions from the end of the episode. Pick an answer to see why it's right.