Next Greater Element Using Stack in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push elements onto the stack.

DSA C

stack[++top] = [1];

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

Atop

Bi

Carr[i]

Dstack[top]

Attempts:

3 left

💡 Hint

Common Mistakes

Pushing the index instead of the element.

Using top instead of arr[i].

We push the current array element arr[i] onto the stack to find its next greater element later.

2fill in blank

medium

Complete the condition to check if the stack is not empty.

DSA C

while (top [1] -1 && stack[top] <= arr[i]) {

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

A>

B==

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '>' to check stack emptiness.

Using '<' which is incorrect here.

The stack is not empty if top > -1.

3fill in blank

hard

Fix the error in assigning the next greater element.

DSA C

result[i] = [1];

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

Aarr[i]

Bstack[top]

C-1

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning arr[i] instead of stack[top].

Assigning -1 incorrectly here.

The next greater element for arr[i] is the top element of the stack.

4fill in blank

hard

Fill both blanks to correctly initialize the stack and result arrays.

DSA C

int stack[[1]], result[[2]];

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

An

Btop

Ci

Darr

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'top' or 'i' as array sizes.

Using 'arr' which is not a size.

Both stack and result arrays should be of size n, the number of elements.

5fill in blank

hard

Fill all three blanks to complete the loop that assigns -1 to elements with no next greater element.

DSA C

for ([1] = 0; [2] < [3]; [2]++) {
    result[i] = -1;
}

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

Ai

Bj

Cn

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables for loop initialization and condition.

Using 'top' instead of 'n' for loop limit.

Use 'i' as the loop variable and 'n' as the limit to assign -1 to all elements without a next greater element.

Practice

1. What does the Next Greater Element (NGE) problem using a stack find for each element in an array?

easy

A. The first greater element to the right of each element

B. The first smaller element to the left of each element

C. The last element in the array

D. The sum of all elements greater than the current element

Solution

1. Step 1: Understand the problem definition

   The Next Greater Element problem asks for the first element to the right of each element that is greater than it.

2. Step 2: Identify the role of the stack

   The stack helps efficiently find this next greater element by storing candidates and popping smaller elements.

3. Final Answer:

   The first greater element to the right of each element -> Option A

4. Quick Check:

   Next Greater Element = first greater right element [OK]

Hint: Look right for the first bigger number using a stack [OK]

Common Mistakes:

• Confusing next greater with next smaller element
• Looking to the left instead of right
• Thinking it sums elements instead of finding one

2. Which of the following C code snippets correctly initializes an empty stack for Next Greater Element problem?

easy

A. int stack[100]; int top = -1;

B. int stack[] = {0}; int top = 0;

C. int *stack; int top = NULL;

D. stack = new int[100]; top = -1;

Solution

1. Step 1: Check valid C array and top initialization

   int stack[100]; int top = -1; declares a fixed size array and sets top to -1 indicating empty stack.

2. Step 2: Identify invalid syntax or semantics in other options

   int stack[] = {0}; int top = 0; initializes top to 0 which means stack is not empty; int *stack; int top = NULL; uses NULL for int top which is invalid; stack = new int[100]; top = -1; uses 'new' which is C++ syntax, not C.

3. Final Answer:

   int stack[100]; int top = -1; -> Option A

4. Quick Check:

   Stack array + top = -1 means empty [OK]

Hint: Use array and top = -1 for empty stack in C [OK]

Common Mistakes:

• Using C++ 'new' keyword in C code
• Initializing top to 0 instead of -1
• Using pointer without allocation

3. What is the output of the Next Greater Element array for input [4, 5, 2, 25] using the stack method?

medium

A. [25, 25, -1, -1]

B. [5, 2, 25, -1]

C. [5, 25, 25, -1]

D. [-1, -1, -1, -1]

Solution

1. Step 1: Process each element to find next greater

   For 4, next greater is 5; for 5, next greater is 25; for 2, next greater is 25; for 25, no greater element, so -1.

2. Step 2: Confirm output array

   The output array is [5, 25, 25, -1].

3. Final Answer:

   [5, 25, 25, -1] -> Option C

4. Quick Check:

   Next greater elements match output [OK]

Hint: Check each element's right side for first bigger number [OK]

Common Mistakes:

• Assigning smaller or wrong next greater values
• Using -1 for elements that do have greater elements
• Confusing order of output

4. Identify the bug in this C code snippet for Next Greater Element using stack:

for (int i = 0; i < n; i++) {
  while (top != -1 && arr[stack[top]] < arr[i]) {
    result[stack[top]] = arr[i];
    top--;
  }
  stack[++top] = i;
}
while (top != -1) {
  result[stack[top]] = 0;
  top--;
}

medium

A. Using post-increment instead of pre-increment on stack index

B. Assigning 0 instead of -1 for elements with no greater element

C. Incorrect loop condition causing infinite loop

D. Not initializing top before loop

Solution

1. Step 1: Check final assignment for elements with no next greater

   The code assigns 0 instead of -1, which is incorrect as per problem definition.

2. Step 2: Verify other parts of code

   Stack index increment and loop conditions are correct; top is assumed initialized before.

3. Final Answer:

   Assigning 0 instead of -1 for elements with no greater element -> Option B

4. Quick Check:

   No greater element should be -1, not 0 [OK]

Hint: Use -1 for no next greater element, not 0 [OK]

Common Mistakes:

• Using 0 instead of -1 for no next greater
• Forgetting to initialize top
• Wrong increment operator on stack

5. Given the array [13, 7, 6, 12], what is the Next Greater Element array using the stack approach? Also, explain why the last element's next greater is -1.

hard

A. [ 7, 12, 13, -1 ]

B. [ -1, 13, 12, -1 ]

C. [ -1, 12, -1, -1 ]

D. [ -1, 12, 12, -1 ]

Solution

1. Step 1: Find next greater for each element

   For 13, no greater element to right, so -1; for 7, next greater is 12; for 6, next greater is 12; for 12, no greater element, so -1.

2. Step 2: Explain last element's next greater

   The last element 12 has no elements to its right, so no next greater element exists, hence -1.

3. Final Answer:

   [ -1, 12, 12, -1 ] -> Option D

4. Quick Check:

   Last element no right neighbor = -1 [OK]

Hint: Last element always has -1 as no elements to right [OK]

Common Mistakes:

• Assigning next greater to last element
• Mixing up next greater for middle elements
• Confusing output order