Given an integer array arr, find, for every position, the nearest element to its left that is strictly greater than it — or -1 if no such element exists. Scanning backward from scratch for every position works but repeats a lot of comparisons unnecessarily. A monotonic stackA stack kept deliberately in sorted order (here, strictly decreasing from bottom to top) by discarding entries that a new arrival makes permanently unreachable as an answer. of strictly decreasing values fixes this in one left-to-right pass: whenever a new element arrives, everything smaller-or-equal sitting on top of the stack can never be anyone's "previous greater" answer again — the new, bigger, closer element would always be found first — so those entries are popped and discarded for good. Whatever survives on top after popping is exactly the answer for the current position.