Visualized DSA Strategies

Episode 2 Hashing: HashSet and HashMap

1 h 2 min · C# · Quiz

Title card of Episode 2: Hashing: HashSet and HashMap

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The complete guide to hashing for coding interviews, in C#. 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; HashSet and Dictionary in C# (Add, TryGetValue, GetValueOrDefault); 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
  • HashSet and Dictionary: Add, Contains, Remove, the indexer, TryGetValue, GetValueOrDefault
  • Seen before? A set, and a set as a window of the last k numbers
  • Look up the complement: Two Sum with a Dictionary
  • Counting with a Dictionary, and with int[26]
  • Grouping by a key, and set operations on two arrays
  • The longest consecutive run: only start where a run starts
  • Designing a set with a random pick: a Dictionary plus a List
  • Five traps: KeyNotFoundException, Add on a repeated key, array keys, GetHashCode, adding while looping

Chapters

  1. 0:00Intro
  2. 0:22How a hash set works
  3. 1:35Four questions hashing answers
  4. 2:03What each structure costs
  5. 2:44HashSet and Dictionary in C#
  6. 4:11Five traps
  7. 5:18Nine hashing types
  8. 5:58Type 1 · Is there a duplicate?
  9. 11:31Type 2 · Duplicates close together
  10. 17:28Type 3 · Two numbers that add up
  11. 23:31Type 4 · Are they anagrams?
  12. 29:22Type 5 · Can the note be written?
  13. 35:20Type 6 · Group the anagrams
  14. 41:05Type 7 · What do two arrays share?
  15. 46:59Type 8 · Longest consecutive run
  16. 53:02Type 9 · A set with a random pick
  17. 59:59Spot the type
  18. 1:00:45Quiz: which type?
  19. 1:01:33Recap

Quiz: test yourself

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