Hash Map

Hash Map / Hash Set

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Use when: O(1) lookup, frequency counting, complement search, grouping, detecting duplicates.

Signal words: two sum, anagram, duplicate, group by, count occurrences, seen before.

Common Uses

Complement Lookup (Two Sum pattern)

seen = {}
for i, num in enumerate(nums):
    complement = target - num
    if complement in seen:
        return [seen[complement], i]
    seen[num] = i

Frequency Count

from collections import Counter
freq = Counter(nums)
# or
freq = {}
for x in nums:
    freq[x] = freq.get(x, 0) + 1

Grouping

from collections import defaultdict
groups = defaultdict(list)
for item in items:
    key = get_key(item)
    groups[key].append(item)

Sliding Window with Hash Map

Track character counts in window; shrink when any count exceeds limit.

Key Questions

• What is the key? What is the value?
• Do I need count or just existence? (Counter vs Set)

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Problems Using This Pattern