Learning/Arrays Hashing

Arrays & Hashing

9 questions. This is the foundation section: almost every later pattern reuses the moves learned here.

The one idea

Spend O(n) memory to eliminate a nested loop.

Nearly every question below has the same shape. The brute force contains an inner loop that is searching for something. Searching is exactly what a hash map makes instant — so you replace the inner loop with a lookup into something you built as you went.

The design work is never "use a HashMap". It's choosing what the key should be. That's what these nine questions drill.

Easy Contains Duplicate LC 217 13 min Easy Valid Anagram LC 242 12 min Easy Two Sum LC 1 13 min Medium Group Anagrams LC 49 14 min Medium Top K Frequent Elements LC 347 16 min Medium Encode and Decode Strings LC 271 14 min Medium Product of Array Except Self LC 238 15 min Medium Valid Sudoku LC 36 17 min Medium Longest Consecutive Sequence LC 128 16 min

Prerequisites from Part 1

Read these before starting:

01 Complexity & Problem Triage you need to read constraints and name a target complexity02 Java Collections Toolkit §1 HashMap, §4 HashSet — especially the compute family (merge, computeIfAbsent, getOrDefault)05 equals, hashCode & Keys questions 4 and 8 fail outright if you use an int[] as a map key06 Arrays & Hashing the pattern summary this section instantiates

How to work this section

For each question:

  1. Read §1 and §2 only, then close the file and attempt the problem cold with a timer (20 minutes for Easy, 35 for Medium).
  2. After your attempt, read §3 and §4 and diff against your own reasoning. The gap is your study material — not the parts you got right.
  3. Rehearse §6 aloud. Then use the counter-questions in §3 as a quiz — for each approach, cover the answers and try to defend it yourself.
  4. Read §7 last: those are the modified-constraint variants.

What to carry forward

By the end of this section you should be able to answer, instantly: