Visualized DSA Strategies

Episode 4 Two Pointers

1 h 3 min · C# · Quiz

Episode 4Two Pointers

▶ This video is coming to YouTube soon.

The complete guide to two pointers for coding interviews, in C#. The four shapes (both ends, read and write, two arrays, fast and slow) and the rule that says which pointer moves; what each approach costs; the C# template for each; 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 two-pointer solution the same way.

What you'll learn

  • Why two pointers turn a double loop into one pass
  • Both ends on sorted data: too big moves r, too small moves l
  • A palindrome check that skips punctuation, in O(1) space
  • Moving the shorter wall: the container with the most water
  • 3Sum: sort, fix one number, two pointers on the rest
  • Read and write pointers to filter an array in place
  • Merging from the back so nothing unread is overwritten
  • Three zones to sort 0, 1 and 2 in one pass
  • Fast and slow pointers to find a repeat, and trapping rain water from both ends

Chapters

  1. 0:00Intro
  2. 0:25Two pointers from both ends
  3. 1:07Two pointers, same direction
  4. 1:41Four shapes of two pointers
  5. 2:27What each approach costs
  6. 3:02Two pointers in C#
  7. 4:17Five traps
  8. 5:12Nine two-pointer types
  9. 5:57Type 1 · Two numbers in a sorted array
  10. 11:38Type 2 · Reads the same both ways
  11. 18:47Type 3 · The biggest container
  12. 24:18Type 4 · Triplets that add to zero
  13. 31:18Type 5 · Remove duplicates in place
  14. 37:19Type 6 · Merge into the first array
  15. 43:09Type 7 · Sort the three colours
  16. 48:58Type 8 · The number that repeats
  17. 55:15Type 9 · How much rain is trapped
  18. 1:00:57Spot the type
  19. 1:01:42Quiz: which type?
  20. 1:02:30Recap

Quiz: test yourself

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