Stacks and Queues

Stack Push/Pop

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

Algorithm

Basic Implementation

basic.c 
#include <stdio.h>

void print_values(int values[], int count) {
    for (int i = 0; i < count; i++) {
        if (i > 0) printf(" -> ");
        printf("%d", values[i]);
    }
    printf("\n");
}

int main(void) {
    int stack[3];
    int top = 0;
    int popped[3];
    int count = 0;
    int values[] = {10, 20, 30};
    for (int i = 0; i < 3; i++) stack[top++] = values[i];
    while (top > 0) popped[count++] = stack[--top];
    print_values(popped, count);
    return 0;
}

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

Complexity

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

Implementation notes

  • Keep the explicit stack/queue operations. Library shortcuts that only produce the final list hide the data-structure behavior this lesson is meant to replay.
  • The final output uses a deterministic a -> b -> c format for cross-language comparison.
top The top is the most recently pushed value.
LIFO A stack removes values in last-in, first-out order.