Lesson 8 — Reverse in place

Two pointers, zero extra memory

In place means changing the list you were given instead of building a new one. To reverse in place, put one pointer (an index variable) at the start (i) and one at the end (j). Swap the two items, move i right and j left, and stop when they meet.

# [1, 2, 3, 4, 5]: swap 1<->5, then 2<->4, 3 stays

This uses no extra list, so the memory cost is O(1) — constant, whatever the size.

Why it matters: on huge data (or tiny devices) you cannot always afford a copy. Two pointers is a pattern you will see again in Lesson 14.