aloalgoTwo Sum Pattern
Two Sum is the quintessential hash map problem. Given an array and a target, find two elements that sum to the target. The pattern extends to many variations.Classic Two Sum
Variations
Two Sum II - Sorted Array
When array is sorted, use two pointers instead of hash map for O(1) space.Two Sum - Count Pairs
Two Sum - Data Structure Design
Three Sum
Four Sum
K Sum (General)
Key Insights
- Unsorted array: Use hash map for O(n) time
- Sorted array: Use two pointers for O(1) space
- Finding indices: Store index in hash map
- Counting pairs: Store count in hash map
- K-sum (k≥3): Reduce to Two Sum with sorting + recursion
- Avoid duplicates: Sort first, skip equal consecutive elements
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