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.