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Arrays & Hashing
Hash Map Fundamentals
The single most important data structure for interviews — O(1) lookups that transform brute-force into optimal.
8 min readLeetCode Patterns for InterviewsCode
The Pattern
Hash maps trade O(n) search for O(1) lookup by storing elements as keys. The core trick: on the first pass, ask 'have I already seen a value that combines with the current one to satisfy the constraint?' instead of nesting a second loop.
Hash map — nums = [2, 7, 11, 15], target = 9
[0]
[1]
7
idx 1
[2]
2
idx 0
[3]
[4]
[5]
11
idx 2
Gotwo-sum-—-the-archetypal-hash-map-problem.go
36 linesLn 1, Col 1Go
Build the map as you scan — when you see a number, check if its complement is already in the map from a previous index.
When to Use
- Count frequencies: how often does each element appear?
- Find complements: given x, does (target − x) exist?
- Group by property: anagram groups, equal sums, same patterns.
- Deduplicate in O(n): set membership check without sorting.
Problems
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