What is Stack?

A stack is a linear data structure in which the insertion of a new element and removal of an existing element takes place at the same end represented as the top of the stack.

To implement the stack, it is required to maintain the pointer to the top of the stack, which is the last element to be inserted because we can access the elements only on the top of the stack.

LIFO (Last In First Out): This strategy states that the element that is inserted last will come out first. You can take a pile of plates kept on top of each other as a real-life example. The plate which we put last is on the top, and since we remove the plate that is at the top, we can say that the plate that was put last comes out first.

Stack

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Top 20 Stack Interview Questions

Question Difficulty Level
Implement a stack using an array. Easy
Reverse a string using a stack. Easy
Check for balanced parentheses in an expression. Easy
Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. Medium
Implement a stack that supports push, pop, top, and retrieving the maximum element in constant time. Medium
Evaluate a postfix expression using a stack. Medium
Implement a queue using stacks. Medium
Implement a stack that supports getMin() in O(1) time and O(1) extra space. Hard
Implement a stack that supports getMax() in O(1) time and O(1) extra space. Hard
Design a stack with a function getMin() that returns the minimum element in the stack in O(1) time. Medium
Sort a stack using only one additional stack and no other data structures. Hard
Implement a stack that supports pop, top, and retrieving the middle element in O(1) time. Hard
Implement two stacks in a single array. Easy
Implement a stack with push, pop, empty, and a constant-time minimum function. Medium
Implement a stack that supports sorting elements in ascending order. Medium
Implement a stack that supports sorting elements in descending order. Medium
Implement a stack that supports the operation "Get the kth element from the stack without popping it." Hard
Implement a stack with the following operations: insert, delete, get_random_element. Hard
Design a stack that supports the following operations: push, pop, top, getMin, retrieve the maximum element, and retrieve the kth maximum element. Hard
Design a stack that supports the following operations: push, pop, top, getMin, retrieve the maximum element, and retrieve the kth maximum element. Hard

Top 10 Frequently Asked Questions on Stacks

  1. Q1. What is a stack?
  2. Ans. A stack is a linear data structure that follows the Last In First Out (LIFO) or First In Last Out (FILO) order. It is used to store and manage data where the element inserted last is the first to be removed.

  3. Q2. How do stacks follow the LIFO principle?
  4. Ans. Stacks follow the LIFO principle because the element that is inserted last is the one that is removed first. This is similar to a stack of plates where the plate added on top is the first to be removed.

  5. Q3. What are the basic operations on a stack?
  6. Ans. The basic stack operations include:

  7. Q4. How is push() different from pop()?
  8. Ans. The push() operation is used to add an item to the stack, increasing its size. The pop() operation, on the other hand, removes the top element from the stack, decreasing its size.

  9. Q5. Explain the significance of the isEmpty() operation.
  10. Ans. The isEmpty() operation checks if the stack is empty. It returns true if the stack has no elements and false if it contains one or more elements. It helps avoid errors when attempting to pop from an empty stack.

  11. Q6. What are the types of stacks, and how do they differ?
  12. Ans. There are two main types of stacks:

  13. Q7. What are the common applications of stacks?
  14. Ans. Stacks have various applications, including:

  15. Q8. How is a stack implemented?
  16. Ans. A stack can be implemented using an array or a linked list. Push and pop operations are used to manipulate the stack. In an array-based implementation, the stack size is fixed, while a linked list-based implementation allows dynamic sizing.

  17. Q9. What is the significance of the top() operation?
  18. Ans. The top() operation returns the top element of the stack without removing it. It allows you to access the element at the top of the stack without altering the stack's state.

  19. Q10. Explain the concept of stack complexity analysis.
  20. Ans. Stack complexity analysis involves evaluating the time complexity of stack operations, such as push, pop, isEmpty, and size. These operations typically have O(1) time complexity, meaning they execute in constant time.

Conclusion

In conclusion, a stack is a fundamental linear data structure that operates on the Last In First Out (LIFO) or First In Last Out (FILO) principle. It is used to manage and store data where the last element inserted is the first to be removed. Stacks are employed in various applications across computer science, from converting infix expressions to postfix expressions, implementing undo-redo features in text editors, to aiding in algorithms like Tower of Hanoi and string reversal.

Stacks can be implemented using arrays or linked lists and are characterized by essential operations such as push, pop, top, isEmpty, and size. The time complexity of these operations is typically O(1), ensuring fast and efficient data manipulation.

Understanding stacks and their applications is crucial in computer science, as they play a vital role in solving a wide range of problems and optimizing various processes.

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