Stack Push/Pop - Perl DSA

Algorithm

The replay uses the same three values in every language, so this Perl 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.

Step 2 - Push 10, then 20, then 30

Each push places the new value above the previous top.

Step 3 - Pop removes 30 first

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

Basic Implementation

                            basic.pl
                        

                            use strict;
use warnings;

sub render { return join(" -> ", @_); }

my @stack;
push @stack, $_ for (10, 20, 30);
my @popped;
push @popped, pop @stack while @stack;
print render(@popped), "\n";

                        

Complexity

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

Implementation notes

my @stack declares the stack as a Perl array with the @ sigil.
The replay starts with an empty stack: [].
Push uses push @stack, $_ for (10, 20, 30), appending each value to the end of the array.
After the pushes, the stack is [10, 20, 30]; the top is the rightmost value, 30.
my @popped records values returned by pop operations.
Pop uses pop @stack, which removes and returns the last array element.
push @popped, pop @stack while @stack repeats while the stack is non-empty; in that condition, @stack is in scalar context and means length.
The first pop returns 30, leaving stack [10, 20] and popped [30].
The remaining pops return 20 and 10, leaving stack [] and popped [30, 20, 10].
render(@popped) joins the popped values with " -> ".
print render(@popped), "\n" outputs 30 -> 20 -> 10.