Framework #10 — Hash map vs set vs sorting: when to use which

HASH MAP vs SET vs SORTING
───────────────────────────
HASH MAP (dict) — need value association:
  □ Count occurrences → Counter / defaultdict(int)
  □ Two sum: complement lookup → {val: index}
  □ Group anagrams → {sorted_key: [words]}
  □ Prefix sum → {running_sum: count}
  Time: O(1) avg lookup, O(n) space

HASH SET — need membership only:
  □ Contains duplicate → set(nums)
  □ Intersection of arrays → set1 & set2
  □ Visited tracking → seen = set()
  □ Cycle detection (Floyd's is O(1) space alt)
  Time: O(1) avg lookup, O(n) space

SORTING — when order matters:
  □ Two pointer after sort → O(n log n)
  □ Merge intervals → sort by start
  □ Meeting rooms → sort events
  □ No extra space needed
  Time: O(n log n), O(1) extra space

DECISION:
  Need key→value?          → hash map
  Need "seen before"?      → hash set
  Need O(1) space?         → sort (if O(n log n) OK)
  Need to preserve order?  → hash map (not sort)
  Streaming / online?      → hash (sort needs all data)