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 TypeScript 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.ts 
function render(values: number[]) {
    return values.join(" -> ");
}
const stack: number[] = [];
for (const value of [10, 20, 30]) {
    stack.push(value);
}
const popped: number[] = [];
while (stack.length > 0) {
    popped.push(stack.pop());
}
console.log(render(popped));

Complexity

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

Implementation notes

  • TypeScript declares stack and popped as number[], and render(values: number[]) formats numeric arrays.
  • Push uses stack.push(value) for 10, 20, and 30, mutating the same array from [] to [10, 20, 30]. The array end is the stack top.
  • Pop runs inside while (stack.length > 0), so the replay never observes an undefined pop, though TypeScript's array pop() type is still possibly undefined in stricter settings.
  • Popped values are appended with popped.push(...). The trace shows stack = [10, 20] with popped = [30], then stack = [] with popped = [30, 20, 10].
  • console.log(render(popped)) prints 30 -> 20 -> 10. Visible allocation is the short input literal, the stack array, the popped array, and the joined output string; mutation is limited to push and pop operations.