What occurs during a pop operation on a stack?

During a pop operation on a stack, the topmost element is removed and returned.

In more detail, a stack is a data structure that follows the Last-In-First-Out (LIFO) principle. This means that the last element that was added to the stack will be the first one to be removed. The pop operation is a fundamental operation in a stack, alongside the push operation which adds an element to the top of the stack.

When a pop operation is performed, the topmost element of the stack is removed. If the stack is implemented as an array, this would mean decrementing the top index. If it's implemented as a linked list, the head node would be removed and the next node would become the new head. In both cases, the removed element is also returned by the pop operation. This is useful because it allows the user to both remove the top element and use its value in one operation.

However, it's important to note that attempting to perform a pop operation on an empty stack will result in an error, often called a stack underflow. This is because there is no element to remove and return. To prevent this, it's good practice to always check if the stack is empty before performing a pop operation.

In terms of time complexity, a pop operation on a stack is generally an O(1) operation. This means that it takes a constant amount of time, regardless of the size of the stack. This is because the pop operation only deals with the top element of the stack and doesn't need to interact with any other elements.

In conclusion, the pop operation is a fundamental part of the stack data structure. It removes and returns the topmost element, following the LIFO principle of the stack. However, care must be taken to avoid a stack underflow by ensuring the stack is not empty before performing a pop operation.