Hashmap Existence / Complement Lookup

The Hashmap Existence / Complement Lookup technique is applied when tackling problems characterized by specific invariants in the problem state or constraints:

• Core Strategy: Use a hashmap or set purely to answer 'have I seen this before' or 'does the complement exist', not to count occurrences.
• When to use: Look for opportunities where repeated re-computation can be replaced by maintaining monotonic properties, state windows, or relational pointers.
• Interview Signal: Demonstrating this pattern shows mastery of Hashing foundations, reducing worst-case algorithmic complexity.