Stack Implementation Using Array in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this stack push and pop sequence?

Consider a stack implemented using an array with a maximum size of 3. The following operations are performed in order:

push(10), push(20), pop(), push(30), push(40), pop(), pop()

What is the printed output of the popped elements?

DSA C

int stack[3];
int top = -1;

void push(int x) {
    if (top == 2) return; // stack full
    stack[++top] = x;
}

int pop() {
    if (top == -1) return -1; // stack empty
    return stack[top--];
}

int main() {
    push(10);
    push(20);
    printf("%d ", pop());
    push(30);
    push(40);
    printf("%d ", pop());
    printf("%d", pop());
    return 0;
}

A 10 30 40

B 10 40 30

C 20 40 30

D 20 30 40

💡 Hint

Remember that pop removes the top element and returns it.

The first pop removes 20 (last pushed), then push 30 and 40 fills the stack. Next pop removes 40, then pop removes 30.

❓ Predict Output

intermediate

What is the value of 'top' after these operations?

Given a stack implemented with an array of size 5, initially empty (top = -1). The operations are:

push(5), push(10), push(15), pop(), push(20), pop(), pop()

What is the value of 'top' after these operations?

DSA C

int stack[5];
int top = -1;

void push(int x) {
    if (top == 4) return;
    stack[++top] = x;
}

void pop() {
    if (top == -1) return;
    top--;
}

int main() {
    push(5);
    push(10);
    push(15);
    pop();
    push(20);
    pop();
    pop();
    printf("%d", top);
    return 0;
}

A 0

B 1

C -1

D 2

💡 Hint

Track each push and pop carefully to update 'top'.

After push(5), top=0; push(10), top=1; push(15), top=2; pop(), top=1; push(20), top=2; pop(), top=1; pop(), top=0.

🔧 Debug

advanced

What error does this stack push function cause?

Identify the error in this push function for a stack implemented using an array of size 3:

void push(int x) { if (top == 3) { printf("Stack Overflow\n"); return; } stack[++top] = x; }

Assume 'top' is initialized to -1.

DSA C

int stack[3];
int top = -1;

void push(int x) {
    if (top == 3) {
        printf("Stack Overflow\n");
        return;
    }
    stack[++top] = x;
}

A Syntax error due to missing semicolon

B No error, works correctly

C Stack underflow error

D Stack overflow check misses last valid index, causing out-of-bounds write

💡 Hint

Array indices go from 0 to size-1.

The condition should be 'top == 2' for size 3, because valid indices are 0,1,2. Checking 'top == 3' allows writing beyond array bounds.

❓ Predict Output

advanced

What is the output after these stack operations?

Given a stack of size 4, initially empty, perform:

push(1), push(2), push(3), pop(), push(4), push(5), pop(), pop(), pop()

Print the popped values in order separated by spaces.

DSA C

int stack[4];
int top = -1;

void push(int x) {
    if (top == 3) return;
    stack[++top] = x;
}

int pop() {
    if (top == -1) return -1;
    return stack[top--];
}

int main() {
    push(1);
    push(2);
    push(3);
    printf("%d ", pop());
    push(4);
    push(5);
    printf("%d ", pop());
    printf("%d ", pop());
    printf("%d", pop());
    return 0;
}

A 3 5 4 2

B 3 4 5 2

C 3 4 5 1

D 3 5 4 1

💡 Hint

Remember push fails silently if stack is full.

Pop 1 removes 3, push 4 succeeds, push 5 succeeds (stack full now). Pop removes 5, pop removes 4, pop removes 2.

🧠 Conceptual

expert

Which option correctly describes the time complexity of stack operations using an array?

Consider a stack implemented using an array with fixed size. What is the time complexity of push and pop operations?

A Push is O(n), pop is O(1)

B Both push and pop have O(1) time complexity

C Push is O(1), pop is O(n)

D Both push and pop have O(n) time complexity

💡 Hint

Think about how many elements need to be moved during push or pop.

Push and pop only change the 'top' index and assign or read one element, so both are constant time O(1).