Look for these signals in the problem:
- "Find if element exists" with requirement: When repeated lookups are needed, hash maps avoid scans each time.
- "Two Sum" style problems: Storing complements in a hash map finds pairs in instead of .
- "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.