Coding

Hashmaps

Asked atBByteridge

1Give yourself 5 minutes

2Answer out loud, not in your head

3Then compare with the answer below

Stuck? Show a way to structure it+

01Clarify whether the task needs membership, counts, grouping, or pair lookup
02Choose a map for values and counts or a set for membership
03Maintain the invariant that the structure represents the processed prefix
04Explain expected versus worst-case lookup cost
05Check duplicates, absent keys, negative values, and key normalization

Reference answer

For “Hashmaps”, begin by asking which relationship must be preserved: presence, frequency, an index, or a group. Use a set for membership and a map when a value must carry a count, first index, latest index, or accumulated result. State the streaming invariant: after processing position i, the structure summarizes exactly the prefix through i, which lets you test a complement or update a count without rescanning. Do not claim hashing makes every task O(1): operations are expected O(1) with a good hash and resizing, while adversarial collisions can be worse. Be deliberate about mutating counts and deleting zero counts. Test repeated values, negative numbers, empty input, large keys, and strings requiring normalization. Space is O(number of distinct keys), and a single pass is expected O(n).

Then expect these follow-ups

What constraint would change your chosen approach?
Tests: constraint adaptation
How would you test the edge cases before implementation?
Tests: implementation extension

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