Stacks and Queues

Stack Push/Pop

Push values onto a stack and pop them back in last-in, first-out order.

Algorithm

The replay uses the same three values in every language, so this Ruby DSA implementation can be compared directly with the rest of the DSA track.

top The top is the most recently pushed value.
LIFO A stack removes values in last-in, first-out order.

Visual walkthrough

The same three values from the trace are shown as stack states. The top cell is the next value a pop removes.

Step 1 - Start empty

There is no top value yet.

Empty stack before any push.top of stack(empty)

Step 2 - Push 10, then 20, then 30

Each push places the new value above the previous top.

After push 10, push 20, push 30: 30 is on top.top -> bottom302010

Step 3 - Pop removes 30 first

The top cell leaves first, so the remaining stack starts with 20.

After one pop: popped is 30; 20 is now on top.top -> bottompopped203010

Basic Implementation

basic.rb 
stack = []
[10, 20, 30].each { |value| stack.push(value) }
popped = []
popped << stack.pop until stack.empty?
puts popped.join(" -> ")

Complexity

  • Time: O(1) per push/pop
  • Space: O(n)

Implementation notes

  • The stack is a Ruby Array, starting as stack = [].
  • Push uses stack.push(value) for 10, 20, and 30, so the top is the array's last element.
  • Pop uses stack.pop, which removes and returns that last element while mutating the same array.
  • popped << stack.pop until stack.empty? records each returned value and stops before an empty pop could return nil.
  • The trace shows LIFO order directly: [10, 20, 30] pops 30 first, then 20, then 10.
  • After all pops, stack is [] and popped is [30, 20, 10].
  • puts popped.join(" -> ") prints 30 -> 20 -> 10.