WARM-UP

The move

Learn how work grows with the input (Big O), then reach for a hash map: it answers "have I seen this?" in one hop, so one pass can replace comparing every pair. Stacks, halving and two runners on a linked list round out the toolbox.

Spot it

You're comparing every pair, matching brackets, searching a sorted list, or walking a linked list.

Cost

One pass with a hash map is O(n) time and O(n) space; halving a sorted list is O(log n).