Arrays & Strings

Clear these cleanly and you have cleared most of a Microsoft coding round. The same handful of names recur loop after loop. A Premium question Characters is the most recurring string problem in the loop. Group Anagrams, Two Sum, and Valid Parentheses show up as warm-ups. A Premium question and the two hard ones, a Premium question and Trapping Rain Water, come out when an interviewer wants a real signal. Microsoft leans easier and more classic than Google or Meta, so the bar is bug-free tested code on a problem you have probably seen before, not an exotic trick.

Two skeletons carry almost the whole family. The first is opposing pointers: start at the two ends of an array and walk inward, letting a simple comparison decide which side to retire. A sliding window walks a right edge across a string while a left edge follows behind. A small map tracks what is currently inside the window, so you know when to grow it and when to shrink it. Both replace an O(n^2) scan of every pair or every substring with a single linear pass. The map is what makes the window pass correct.