When to Use

Look for these signals in the problem:

• "Find if element exists" with $O(1)$ requirement: When repeated lookups are needed, hash maps avoid $O(n)$ scans each time.
• "Two Sum" style problems: Storing complements in a hash map finds pairs in $O(n)$ instead of $O(n^2)$.
• "Count frequency" of elements: Hash maps naturally aggregate counts as you iterate through data.
• "Group by" some property: Use transformed keys (like sorted strings for anagrams) to group related elements.
• "First/last occurrence" tracking: Store indices as values to remember positions of elements.