Given an integer array arr, find, for every position, the value of the nearest element strictly smaller than it to its left — or -1 if no such element exists. Scanning backward from every position independently works but repeats a lot of comparisons that a smarter approach can skip entirely. The key insight: once a smaller value appears, every larger value sitting behind it becomes permanently irrelevant — no future position will ever "see past" the smaller, closer value to reach them. A monotonic stackMonotonic StackA stack maintained so its values stay in strictly increasing (or decreasing) order from bottom to top. Before pushing a new value, anything on top that would break that order gets popped off first — those popped values are provably never useful again. exploits exactly that: keep only values in increasing order, discarding anything that a smaller arrival has made obsolete, and whatever survives on top at each step is precisely the answer.