Min Stack: Design a Stack with O(1) Minimum

A monotonic stack is a stack that maintains elements in either increasing or decreasing order. It is a powerful technique for solving problems that involve finding the next greater element, next smaller element, or calculating areas and spans. For this problem, we maintain a monotonic stack and process elements one by one. When a new element violates the monotonic property, we pop elements from the stack and process them — the current element is the answer for those popped elements. After proces