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 Python 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.py 
stack = []
for value in [10, 20, 30]:
    stack.append(value)
popped = []
while stack:
    popped.append(stack.pop())
print(" -> ".join(str(x) for x in popped))

Complexity

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

Implementation notes

  • Python uses a plain list as the stack, with the right end as the top. stack.append(value) mutates that list in place to push, and stack.pop() removes and returns the most recently pushed value.
  • popped.append(stack.pop()) transfers the returned int reference into a separate output list while shrinking the stack. Both end operations are O(1) amortized for Python lists.
  • The persistent containers are just stack and popped; normal Python reference management owns their lifetime once no references remain. The trace shows [10, 20, 30] popping as 30, then 20, then 10.