← DSA notes

Pattern

Hash Map Lookup

Topic

When to use

Need O(1) average lookup to check "have I seen this before" or "what index/count does this value map to." Trade space for a single linear pass instead of nested loops — Two Sum, duplicate checks, frequency counts.

Template code

def hash_map_lookup(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        complement = target - num
        if complement in seen:
            return [seen[complement], i]
        seen[num] = i
    return [-1, -1]

Problems using this pattern

See problems tagged Hash Map Lookup.

Problems