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Stack

A Stack is a fundamental data structure that follows the Last-In-First-Out (LIFO) principle. It's a collection of elements where each element is added and removed from the top of the stack.

Key Characteristics:

  • Last-In-First-Out (LIFO): The last element added to the stack will be the first one to be removed.
  • Top: The top element of the stack is the most recently added or removed element.
  • Push: Adding an element to the stack, which moves the top element up one position.
  • Pop: Removing the top element from the stack.

Operations:

  • Push (or Enqueue): Add an element to the top of the stack.
  • Pop (or Dequeue): Remove the top element from the stack and return it.
  • Peek: Return the top element without removing it from the stack.

Implementation

Python Implementation

# Stack Implementation Using List
stack = []

# Push Elements onto the Stack
stack.append(10)
stack.append(20)
stack.append(30)
print("Stack after pushing:", stack)  # Output: [10, 20, 30]

# Pop Elements from the Stack
print("Popped:", stack.pop())  # Output: 30
print("Stack after popping:", stack)  # Output: [10, 20]

# Peek at the Top Element
print("Top element:", stack[-1])  # Output: 20

# Check if the Stack is Empty
print("Is stack empty?", len(stack) == 0)  # Output: False

Run Time Complexity:

  • Push operation: O(1) (constant time)
  • Pop operation: O(1) (constant time)
  • Peek operation: O(1) (constant time)

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