Min Stack Design in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the min stack's top pointer.

DSA C

typedef struct {
    int *stack;
    int *minStack;
    int top;
    int minTop;
} MinStack;

void initMinStack(MinStack *s) {
    s->top = [1];
    s->minTop = -1;
}

Drag options to blanks, or click blank then click option'

A-1

B0

C1

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing top to 0 causes off-by-one errors.

Using NULL for an integer top index is invalid.

The top index should start at -1 to indicate the stack is empty.

2fill in blank

medium

Complete the code to push a value onto the main stack.

DSA C

void push(MinStack *s, int val) {
    s->stack[++[1]] = val;
}

Drag options to blanks, or click blank then click option'

AminTop

Btop

Cstack

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Using minTop instead of top pushes to the wrong stack.

Not incrementing top causes overwriting the same position.

Incrementing top before assignment places the value correctly on the stack.

3fill in blank

hard

Fix the error in the condition to update the min stack when pushing a new value.

DSA C

if (s->minTop == -1 || val [1] s->minStack[s->minTop]) {
    s->minStack[++s->minTop] = val;
}

Drag options to blanks, or click blank then click option'

A>=

B>

C==

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' causes incorrect min tracking.

Using '==' misses smaller values.

We push to minStack if val is less than or equal to current min, so use '<='.

4fill in blank

hard

Fill both blanks to correctly pop from the main stack and update the min stack if needed.

DSA C

int pop(MinStack *s) {
    int val = s->stack[s->[1]--];
    if (val == s->minStack[s->[2]]) {
        s->minTop--;
    }
    return val;
}

Drag options to blanks, or click blank then click option'

Atop

BminTop

Cstack

Dval

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up top and minTop indices.

Not decrementing top or minTop correctly.

Pop from main stack using top, and compare with minStack at minTop.

5fill in blank

hard

Fill all three blanks to return the current minimum value from the min stack.

DSA C

int getMin(MinStack *s) {
    if (s->[1] == -1) {
        return [2]; // empty stack case
    }
    return s->minStack[s->[3]];
}

Drag options to blanks, or click blank then click option'

AminTop

B-1

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Checking top instead of minTop for empty.

Returning wrong index from minStack.

Check if minTop is -1 (empty), return -1 as error, else return minStack[minTop].

Practice

1. What is the main purpose of a Min Stack data structure?

easy

A. To reverse the order of elements

B. To retrieve the minimum element in constant time

C. To store elements in a queue fashion

D. To sort elements in ascending order

Solution

1. Step 1: Understand Min Stack functionality

   A Min Stack is designed to allow retrieval of the minimum element quickly, usually in constant time.

2. Step 2: Compare with other options

   Sorting or queue behavior is not the main goal of a Min Stack; it focuses on minimum element retrieval.

3. Final Answer:

   To retrieve the minimum element in constant time -> Option B

4. Quick Check:

   Min Stack = fast minimum retrieval [OK]

Hint: Min Stack always tracks minimum element fast [OK]

Common Mistakes:

• Confusing Min Stack with sorting algorithms
• Thinking it behaves like a queue
• Assuming it reverses elements

2. Which of the following is the correct way to declare a stack node structure for a Min Stack in C?

easy

A. typedef struct Node { int value; struct Node* next; int min; } Node;

B. typedef struct Node { int value; int min; } Node;

C. typedef struct Node { int value; struct Node* prev; } Node;

D. typedef struct Node { int value; struct Node* next; } Node;

Solution

1. Step 1: Identify required fields for Min Stack node

   A Min Stack node needs to store the value, a pointer to the next node, and the minimum value at that node.

2. Step 2: Match with options

   typedef struct Node { int value; struct Node* next; int min; } Node; includes value, next pointer, and min field, which is correct for Min Stack implementation.

3. Final Answer:

   typedef struct Node { int value; struct Node* next; int min; } Node; -> Option A

4. Quick Check:

   Node stores value, next, and min [OK]

Hint: Min Stack node needs value, next pointer, and min [OK]

Common Mistakes:

• Omitting the min field in node
• Using prev pointer instead of next
• Not using struct pointer for next

3. Consider the following C code snippet for a Min Stack push operation:

void push(Node** top, int x) {
    Node* newNode = malloc(sizeof(Node));
    newNode->value = x;
    if (*top == NULL) {
        newNode->min = x;
    } else {
        newNode->min = (x < (*top)->min) ? x : (*top)->min;
    }
    newNode->next = *top;
    *top = newNode;
}

What will be the minimum value after pushing elements 3, 5, 2 in this order on an empty stack?

medium

A. 2

B. 3

C. 5

D. Error

Solution

1. Step 1: Push 3 on empty stack

   Stack top is NULL, so min = 3.

2. Step 2: Push 5

   Compare 5 with current min 3; min remains 3.

3. Step 3: Push 2

   Compare 2 with current min 3; min updates to 2.

4. Final Answer:

   2 -> Option A

5. Quick Check:

   Minimum after pushes = 2 [OK]

Hint: Min updates only if new value is smaller [OK]

Common Mistakes:

• Not updating min on push
• Confusing min with last pushed value
• Assuming min resets on each push

4. The following C function is intended to pop an element from a Min Stack. Identify the error:

int pop(Node** top) {
    if (*top == NULL) return -1;
    int val = (*top)->value;
    free(*top);
    *top = (*top)->next;
    return val;
}

medium

A. Not checking if stack is empty

B. Returning value before freeing node

C. Accessing (*top)->next after freeing *top

D. Missing return statement

Solution

1. Step 1: Analyze order of operations

   The code frees *top, then accesses (*top)->next, which is invalid after free.

2. Step 2: Correct order

   Should save next pointer before freeing current node to avoid invalid access.

3. Final Answer:

   Accessing (*top)->next after freeing *top -> Option C

4. Quick Check:

   Free before next access causes error [OK]

Hint: Save next pointer before freeing node [OK]

Common Mistakes:

• Freeing node before accessing next
• Ignoring NULL check
• Returning value after free without saving

5. You want to design a Min Stack that supports push, pop, and getMin all in O(1) time using two stacks. Which approach correctly implements getMin?

hard

A. Use a queue to track minimum values

B. Sort the main stack on every push to keep minimum on top

C. Traverse the main stack on every getMin call to find minimum

D. Maintain a second stack that stores the minimum values; push new min or duplicate current min on push

Solution

1. Step 1: Understand two-stack Min Stack design

   One stack stores all values; the second stack keeps track of minimums for O(1) retrieval.

2. Step 2: Evaluate options

   Maintain a second stack that stores the minimum values; push new min or duplicate current min on push correctly describes pushing new minimum or current minimum duplicate onto second stack.

3. Final Answer:

   Maintain a second stack that stores the minimum values; push new min or duplicate current min on push -> Option D

4. Quick Check:

   Two stacks: one for values, one for mins [OK]

Hint: Use second stack to track mins for O(1) getMin [OK]

Common Mistakes:

• Sorting stack on every push (slow)
• Searching stack on getMin (not O(1))
• Using queue instead of stack