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📈 Sorting

Sort first, ask questions later. Many "hard" array problems become trivial once sorted. Know the sorts well enough to choose, and know when to use a custom comparator.

This category contains 21 problems. Use the patterns below to recognize what's being asked, then jump to the problem list at the bottom.

🧠 Key Patterns​

  • Sort + Two Pointers — Pair/triplet sum, closest target.
  • Counting / Bucket Sort — Integer values in known range → O(n)O(n).
  • Custom Comparator — Sort by frequency, by digit, "largest number" trick.
  • Merge Sort — Count inversions, external sorting, stable sort.
  • Quickselect — O(n)O(n) average for k-th smallest.
  • Heap Sort / Top-K — Partial sort when full sort is overkill.

⚠️ Common Pitfalls​

  • Default sort in C# (Array.Sort) is not stable; List<T>.Sort is also not stable. Use OrderBy (LINQ) for stable.
  • Comparator returning subtraction can overflow with int.MinValue.

📚 Study Resources​

📺 Videos​

📖 Books​

  • Introduction to Algorithms (CLRS) — Ch. 2, 6–8
  • Algorithms — Sedgewick — Ch. 2

🌐 Articles & References​

💻 All Sorting Problems​

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