Monotonic Stack

What it is

A monotonic stack is a stack kept in order.

When a new element breaks that order, you pop until the property is restored.

When to use it

It usually appears when the problem asks for:

• next greater element
• next smaller element
• days until a warmer temperature
• spans

Common mistake

The classic mistake is treating it like an exotic technique.

In practice, it is just an organized way to say:

“some older elements stopped mattering after this one arrived.”

Better question

Before using it, ask:

1. does the problem compare each element to future or past neighbors?
2. is there a group of elements that loses relevance when another appears?
3. am I repeating too many comparisons in nested loops?