Two Pointers

Start one pointer at each end of a sorted array or string and move them toward each other, discarding a whole set of candidates with every step, so one O(n) pass replaces checking all pairs.

Move one pointer one step and another two steps at a time: if there is a cycle the fast one laps the slow one, and if there is none the slow one ends at the middle when the fast one hits the end.

One pointer reads every element while a second marks where the next kept element goes, so you filter or compact an array in place in a single O(n) pass.

Sort the array, fix one element, and find the rest with two pointers: two sum becomes O(n), three sum O(n^2), and duplicates are skipped by comparing neighbours.

Walk two sorted sequences with one pointer each, always taking the smaller front element, to produce one sorted result in O(n + m).