Next Greater Element Using Stack in DSA Python - Build from Scratch

Next Greater Element Using Stack

📖 Scenario: Imagine you have a list of daily temperatures. For each day, you want to find the next day when the temperature will be higher. This helps you plan your activities better.

🎯 Goal: You will build a program that finds the next greater element for each number in a list using a stack. The next greater element is the first number to the right that is bigger than the current number.

📋 What You'll Learn

Create a list called numbers with exact values [4, 5, 2, 25]

Create an empty list called result of the same length as numbers filled with -1

Use a stack (list) called stack to help find the next greater elements

Use a for loop with variable i to iterate over the indexes of numbers

Print the result list at the end

💡 Why This Matters

🌍 Real World

Finding the next greater element is useful in stock price analysis, weather forecasting, and many other areas where you want to know the next time a value increases.

💼 Career

Understanding stacks and how to use them to solve problems like next greater element is important for software development roles, especially those involving algorithms and data processing.

1

Create the list of numbers

Create a list called numbers with these exact values: [4, 5, 2, 25]

DSA Python

# Create the list called numbers with values [4, 5, 2, 25]
# Your code here

Hint

Use square brackets and commas to create the list.

2

Prepare the result list and stack

Create a list called result with the same length as numbers filled with -1. Also create an empty list called stack.

DSA Python

numbers = [4, 5, 2, 25]
# Create result list filled with -1 and empty stack list
# Your code here

Hint

Use [-1] * len(numbers) to create the result list.

3

Find next greater elements using a stack

Use a for loop with variable i to iterate over the indexes of numbers. Inside the loop, use a while loop to check if the current number is greater than the number at the index on top of the stack. If yes, update result at that index with the current number and pop from stack. After the while loop, append i to stack.

DSA Python

numbers = [4, 5, 2, 25]
result = [-1] * len(numbers)
stack = []
# Use for loop with i in range(len(numbers)) and while loop to find next greater elements
# Your code here

Hint

Use stack[-1] to get the last element in the stack.

4

Print the result list

Print the result list to show the next greater elements for each number in numbers.

DSA Python

numbers = [4, 5, 2, 25]
result = [-1] * len(numbers)
stack = []

for i in range(len(numbers)):
    while stack and numbers[i] > numbers[stack[-1]]:
        index = stack.pop()
        result[index] = numbers[i]
    stack.append(i)

# Print the result list
# Your code here

Hint

The output should be a list showing the next greater element for each number.

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