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 Swift 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.swift 
func render(_ values: [Int]) -> String {
    values.map(String.init).joined(separator: " -> ")
}

var stack: [Int] = []
for value in [10, 20, 30] { stack.append(value) }
var popped: [Int] = []
while !stack.isEmpty { popped.append(stack.removeLast()) }
print(render(popped))

Complexity

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

Implementation notes

  • var stack: [Int] = [] is a mutable Swift array used as stack storage, with the back of the array acting as the top.
  • The push loop for value in [10, 20, 30] { stack.append(value) } appends each Int in order, producing [10, 20, 30].
  • var popped: [Int] = [] records pop order separately from the remaining stack.
  • The loop guard while !stack.isEmpty prevents calling removeLast() on an empty array, so this source has no optional return or sentinel path.
  • popped.append(stack.removeLast()) removes from the array end, so 30 is recorded before 20 and 10.
  • The trace shows the stack moving from [] to [10, 20, 30], then the first pop producing popped = [30] and stack = [10, 20], and finally popped = [30, 20, 10] with stack = [].
  • render(_:) maps each Int to String and joins with " -> ", so print(render(popped)) outputs 30 -> 20 -> 10.