Stack & Queue Problems Master List 🎯

Welcome to our comprehensive guide on Stack and Queue problems! 📝

In this lesson, we'll dive deep into these essential data structures, learn their real-world applications, and solve various problems to strengthen your understanding. Let's get started!

What are Stacks and Queues? 💡

Stacks

A Stack is a Linear data structure that follows the LIFO (Last In, First Out) principle. It's like a pile of books where the last book you put is the first one you take out.

python

# Python Stack Example
from collections import deque

stack = deque()
stack.append(1)  # Add element to the end
stack.append(2)  # Add element to the end
stack.popleft()   # Remove and return the first element (last one added)

Queues

A Queue is a Linear data structure that follows the FIFO (First In, First Out) principle. It's like a line at a grocery store, where the first person in line is the first to be served.

python

# Python Queue Example
from collections import deque

queue = deque()
queue.append(1)  # Add element to the end
queue.append(2)  # Add element to the end
queue.popleft()   # Remove and return the first element (first one added)

Problems 📝

Stack Problems

Next Greater Element Given an array, find the next greater element for each element. If there is no next greater element, consider the next greater element as -1.

python
def nextGreaterElement(arr, n): stack, res = [], [-1] * n for i in range(n): while stack and stack[-1] < arr[i]: stack.pop() if stack: res[stack[-1]] = arr[i] stack.append(arr[i]) return res

Queue Problems

Sliding Window Maximum Given an array and a sliding window (k), find the maximum element in every contiguous subarray of size 'k'.

python
def maxSlidingWindow(arr, k): deque, res = deque(), [] for i in range(len(arr)): while deque and deque[-1] < arr[i]: deque.pop() deque.append(arr[i]) if i >= k - 1: res.append(deque[0]) if deque[0] == arr[i - k + 1]: deque.popleft() return res

With these problems, you'll get hands-on experience with Stacks and Queues, making it easier to understand their practical applications. Happy coding! 💻🚀