Validate Stack Sequences

Validate Stack Sequences

What is this problem about?

The "Validate Stack Sequences" interview question gives you two sequences: pushed and popped. You must determine if this popped sequence could have resulted from a series of push and pop operations on an initially empty stack using the pushed sequence. Each element in the pushed sequence is distinct.

Why is this asked in interviews?

Companies like Microsoft and Amazon use the "Validate Stack Sequences" coding problem to test a candidate's ability to simulate data structures. It assesses your "Stack interview pattern" knowledge and your ability to manage pointers and state during a simulation. It’s a great way to see if a candidate can visualize the LIFO (Last-In-First-Out) nature of a stack.

Algorithmic pattern used

The best approach is a "Greedy Simulation." You use a real stack (or a list acting as a stack) and iterate through the pushed array. For each element you push, you check if the top of the stack matches the current element in the popped array. If it does, you pop it and move to the next element in the popped array, repeating this as long as there is a match.

Example explanation

pushed = [1, 2, 3, 4], popped = [3, 4, 2, 1]

1. Push 1: Stack [1].
2. Push 2: Stack [1, 2].
3. Push 3: Stack [1, 2, 3]. Top (3) matches popped[0] (3). Pop it. Stack [1, 2].
4. Push 4: Stack [1, 2, 4]. Top (4) matches popped[1] (4). Pop it. Stack [1, 2].
5. Top (2) matches popped[2] (2). Pop it. Stack [1].
6. Top (1) matches popped[3] (1). Pop it. Stack [].
7. Result: True (Stack is empty and all elements popped).

Common mistakes candidates make

A common mistake is trying to solve this with complex logic without using an actual stack for simulation. Another error is not checking if the stack is empty before accessing its top element. Candidates also sometimes fail to realize that the "pop" step must be a loop, as multiple elements might be poppable after a single "push."

Interview preparation tip

When you hear the word "Simulation" in a problem description, your first instinct should be to use the actual data structure mentioned. For the "Validate Stack Sequences" coding problem, using a real stack makes the code follow the problem's logic perfectly, making it much easier to reason about and debug.