Two Sum → Two Pointer → Three/Four Sum

• Two Sum with HashMap is O(n) time, O(n) space
• Two Pointer needs sorted array, but uses O(1) space — good answer when interviewer asks "can you do better on space?"
• Three Sum → fix i, use two pointers for the rest
• Four Sum → fix i and j, use two pointers for the rest
• Same pattern, just more anchor points

Full breakdown here.