Pattern: Hash Map for Complement Lookup 1. Create an empty hash map to store {value: index} 2. Iterate through the array once: - Calculate complement = target - nums[i] - If complement exists in map, return [map[complement], i] - Otherwise, store nums[i] with its index 3. Key insight: We only need one pass because we check before adding Edge cases: - Same element used twice (handle by checking before adding) - Negative numbers (works fine with hash map) Time: O(n) - single pass Space: O(n) - hash map storage