Remember What You Have Seen

Maps, Sets and plain objects look things up in O(1) on average, so many problems become fast when you remember what you have already seen. Count with a Map. Group into an object of arrays. Find two numbers that add up to a target in one pass by storing each number as you go, instead of checking every pair.

How it reads

• The Map counts every word in a single pass
• ||= creates the array the first time a length appears
• twoSum asks "have I seen the number I need?" instead of trying every pair

Cloud tip: Storing what you learn so you never recompute it is the most useful algorithm trick there is.