Next Greater Element Using Stack in DSA Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Next Greater Element (NGE) for an element in an array?

The Next Greater Element for an element is the first element to the right of it in the array that is greater than it. If no such element exists, the NGE is -1.

intermediate

Why do we use a stack to find the Next Greater Element efficiently?

A stack helps keep track of elements for which we haven't found the NGE yet. It allows us to compare the current element with the top of the stack and find NGEs in a single pass, making the solution efficient.

intermediate

What is the time complexity of the Next Greater Element algorithm using a stack?

The time complexity is O(n) because each element is pushed and popped from the stack at most once during the single pass through the array.

intermediate

In the Next Greater Element algorithm, what do you do when the current element is greater than the element at the top of the stack?

You pop the top element from the stack and assign the current element as its Next Greater Element. Repeat this until the stack is empty or the top element is greater than or equal to the current element.

beginner

What value do you assign as the Next Greater Element if no greater element exists to the right?

You assign -1 to indicate that there is no Next Greater Element for that position.

What data structure is best suited for solving the Next Greater Element problem efficiently?

A Hash Map

B Queue

C Stack

D Linked List

A stack allows us to keep track of elements waiting for their Next Greater Element and process them efficiently in one pass.

What is the Next Greater Element for the last element in any array?

A -1

B The element itself

C The first element

D The previous element

The last element has no elements to its right, so its Next Greater Element is always -1.

If the current element is smaller than the top of the stack, what should you do in the NGE algorithm?

A Push the current element onto the stack

B Pop the stack

C Ignore the current element

D Replace the top of the stack

If the current element is smaller, push it onto the stack to find its NGE later.

What is the time complexity of the Next Greater Element algorithm using a stack?

A O(n^2)

B O(log n)

C O(n log n)

D O(n)

Each element is pushed and popped at most once, so the algorithm runs in linear time O(n).

What does the stack store during the Next Greater Element algorithm?

A All elements in sorted order

B Indices or values of elements waiting for their NGE

C Only the maximum element

D Elements that already have their NGE

The stack stores elements (or their indices) for which we haven't found the Next Greater Element yet.

Explain step-by-step how to find the Next Greater Element for each element in an array using a stack.

Think about how the stack helps track elements waiting for their NGE.

Why is the Next Greater Element algorithm using a stack more efficient than a brute force approach?

Compare how many times each element is checked in both methods.