Two-pointer walks, in and out

Two pointers marching from opposite ends, or in the same direction at different speeds, is the second-most-recycled interview pattern after hash-map lookup. Reverse in place, palindromes, sorted-array pair sums — same three-line skeleton every time.

By the end you should be able to spot the shape from the problem statement alone: "in-place", "sorted array", "reverse" — all two-pointer tells.

Follow the order

1. 01

   problem

   Reverse String

   Reverse the input string. In-place is preferred where the language allows.

   Warm-up — the simplest instance. Swap and shrink until the pointers meet.

   ++++
2. 02

   problem

   Valid Parentheses

   Given a string containing only (){}[], determine whether every opener has its matching closer, correctly nested.

   Same intuition, different data structure. Stack the openers, pop on closers.

   ++++