Next Greater Element Using Stack in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push the current element onto the stack.

DSA Python

stack.[1](arr[i])

Drag options to blanks, or click blank then click option'

Apop

Bappend

Cpush

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of append.

Using push which is not a Python list method.

In Python, to add an element to the end of a list (used as a stack), we use append().

2fill in blank

medium

Complete the code to pop the top element from the stack.

DSA Python

top = stack.[1]()

Drag options to blanks, or click blank then click option'

Apop

Bremove

Cdelete

Ddiscard

Attempts:

3 left

💡 Hint

Common Mistakes

Using remove which removes by value, not by position.

Using delete which is not a list method.

pop() removes and returns the last element from the list, which is the top of the stack.

3fill in blank

hard

Fix the error in the while loop condition to check if stack is not empty.

DSA Python

while stack and stack[-1] [1] arr[i]:

Drag options to blanks, or click blank then click option'

A<

B>

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using greater than or equal operators which reverse the logic.

Using less than or equal which includes equal values incorrectly.

We want to pop elements smaller than the current element, so the condition is stack[-1] < arr[i].

4fill in blank

hard

Fill both blanks to complete the dictionary comprehension for the next greater element.

DSA Python

result = {num: [1] for num in arr if num [2] -1}

Drag options to blanks, or click blank then click option'

Astack[-1]

B==

C!=

Darr[-1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' which would only include the last element.

Using arr[-1] which is the last element of the array, not the next greater element.

We map each number to the top of the stack (next greater element), excluding the last element which has no next greater element.

5fill in blank

hard

Fill all three blanks to complete the function that finds the next greater element for each item in the list.

DSA Python

def next_greater_element(arr):
    stack = []
    result = {}
    for i in range([1]):
        while stack and stack[-1] < arr[i]:
            result[stack.pop()] = arr[i]
        stack.[2](arr[i])
    for num in stack:
        result[num] = [3]
    return result

Drag options to blanks, or click blank then click option'

Alen(arr)

Bappend

C-1

DNone

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect loop range.

Using push instead of append.

Assigning -1 instead of None for no next greater element.

We loop through the entire array length, append elements to the stack, and assign None to elements with no next greater element.

Practice

1. What does the Next Greater Element problem using a stack help you find for each element in an array?

easy

A. The first smaller element to the left of each element

B. The first greater element to the right of each element

C. The sum of all elements greater than the current element

D. The index of the largest element in the array

Solution

1. Step 1: Understand the problem goal

   The Next Greater Element problem asks to find, for each element, the first element to its right that is greater than it.

2. Step 2: Recognize the stack usage

   A stack helps efficiently track elements waiting for their next greater element to appear on the right.

3. Final Answer:

   The first greater element to the right of each element -> Option B

4. Quick Check:

   Next Greater Element = first bigger right element [OK]

Hint: Look right for first bigger number using stack [OK]

Common Mistakes:

• Confusing next greater with next smaller element
• Looking to the left instead of right
• Thinking it finds the largest element overall

2. Which of the following is the correct way to initialize an empty stack in Python for solving Next Greater Element?

easy

A. stack = []

B. stack = {}

C. stack = ()

D. stack = None

Solution

1. Step 1: Identify stack data structure in Python

   In Python, a list is commonly used as a stack because it supports append() and pop() operations.

2. Step 2: Check initialization syntax

   An empty list is created with square brackets []. Curly braces {} create a dictionary, parentheses () create a tuple, and None is a null value.

3. Final Answer:

   stack = [] -> Option A

4. Quick Check:

   Empty stack = empty list [] [OK]

Hint: Use list [] for stack in Python [OK]

Common Mistakes:

• Using {} which creates a dictionary
• Using () which creates a tuple
• Assigning None instead of a list

3. What is the output of the Next Greater Element for the array [4, 5, 2, 25]?

medium

A. [25, 25, -1, -1]

B. [5, -1, 25, -1]

C. [5, 25, 25, -1]

D. [-1, -1, -1, -1]

Solution

1. Step 1: Find next greater for each element

   For 4, next greater is 5; for 5, next greater is 25; for 2, next greater is 25; for 25, no greater element, so -1.

2. Step 2: Write the output array

   The output array is [5, 25, 25, -1].

3. Final Answer:

   [5, 25, 25, -1] -> Option C

4. Quick Check:

   Check each element's next bigger right = output [OK]

Hint: Scan right for bigger number or -1 if none [OK]

Common Mistakes:

• Putting -1 too early for elements that have bigger right
• Confusing order of output elements
• Missing last element's -1

4. Given the code snippet below, what is the error?

def next_greater(arr):
    stack = []
    result = [-1] * len(arr)
    for i in range(len(arr)):
        while stack and arr[stack[-1]] < arr[i]:
            result[stack.pop()] = arr[i]
        stack.append(i)
    return result

print(next_greater([2, 7, 3, 5, 4, 6, 8]))

medium

A. No error, code runs correctly

B. IndexError due to wrong stack usage

C. SyntaxError because of missing colon

D. TypeError due to wrong list initialization

Solution

1. Step 1: Analyze the code logic

   The code uses a stack to store indices and updates the result list with next greater elements correctly.

2. Step 2: Check for syntax and runtime errors

   All syntax is correct, stack usage is proper, and list initialization is valid.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Stack and result used correctly, no errors [OK]

Hint: Trace stack push/pop carefully to spot errors [OK]

Common Mistakes:

• Assuming index errors without checking stack conditions
• Confusing syntax errors with logic errors
• Misreading list initialization syntax

5. You have an array [13, 7, 6, 12]. Using the Next Greater Element approach with a stack, what will be the final output array?

hard

A. [7, 12, 12, -1]

B. [13, 12, 12, -1]

C. [-1, -1, 12, -1]

D. [-1, 12, 12, -1]

Solution

1. Step 1: Find next greater element for each index

   For 13, no greater element to right, so -1; for 7, next greater is 12; for 6, next greater is 12; for 12, no greater element, so -1.

2. Step 2: Write the output array

   The output is [-1, 12, 12, -1].

3. Final Answer:

   [-1, 12, 12, -1] -> Option D

4. Quick Check:

   Check each element's next bigger right = output [OK]

Hint: Remember last element always -1 if no bigger right [OK]

Common Mistakes:

• Assuming first element has a bigger next element incorrectly
• Mixing values with indices in output
• Forgetting to put -1 when no greater element exists