Given a stack of integers, reverse its contents completely, using only push, pop, top, and isEmpty — no other explicit data structure, and no loops for the reversal logic itself. The recursive technique builds directly on "insert a value at the bottom of a stack": pop the top element, recursively reverse whatever remains beneath it, then insert the popped element at the very bottom of that now-reversed remainder. Reaching the empty stack (the base case) is where the unwinding starts — each level restores its held-aside value to the bottom, one at a time, until the whole stack has flipped end to end. It's worth being upfront about the cost of this elegance: because inserting at the bottom is itself an O(n) recursive walk, and it happens once per level of the outer recursion, the total work comes out to O(n²) — genuinely slower than simply copying into a second array, which is the better choice whenever the "no auxiliary structure" constraint doesn't actually apply.