Next Greater Element Using Stack in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push the current element onto the stack.

DSA C#

stack.[1](arr[i]);

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

APop

BClear

CPeek

DPush

Attempts:

3 left

💡 Hint

Common Mistakes

Using Pop instead of Push removes elements instead of adding.

Using Peek only views the top element without adding.

We use Push to add the current element to the stack for future comparisons.

2fill in blank

medium

Complete the code to check if the stack is not empty before accessing its top element.

DSA C#

while (stack.Count > 0 && stack.Peek() [1] arr[i])

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

A<

B>

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' reverses the logic and causes incorrect popping.

Using '==' or '!=' does not help find greater elements.

We want to pop elements smaller than the current element, so we check if top of stack is less than current.

3fill in blank

hard

Fix the error in assigning the next greater element when the stack is empty.

DSA C#

result[i] = stack.Count == 0 ? [1] : stack.Peek();

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

Aarr[i]

B0

C-1

Dint.MaxValue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or arr[i] can give wrong results.

Using int.MaxValue is not standard for this problem.

When no greater element exists, we assign -1 as per convention.

4fill in blank

hard

Fill both blanks to complete the for loop iterating from the end to the start of the array.

DSA C#

for (int i = arr.Length - 1; i [1]= 0; i[2])

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

A>

B>=

C--

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' excludes index 0.

Using '++' moves forward, not backward.

We start from the last index and move backwards, so i >= 0 and i-- are correct.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps each element to its next greater element.

DSA C#

var nextGreater = new Dictionary<int, int>();
for (int i = 0; i [1] arr.Length; i[2])
{
    nextGreater[arr[i]] = [3];
}

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

A<

B++

Cresult[i]

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '--' or '>=' causes wrong loop direction.

Assigning arr[i] instead of result[i] gives wrong mapping.

We loop forward with i < arr.Length and i++ and assign result[i] as the next greater element.

Practice

1. What does the Next Greater Element 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 sum of all elements greater than the current element

D. The index of the maximum element in the array

Solution

1. Step 1: Understand the problem goal

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

2. Step 2: Identify the role of the stack

   A stack helps track elements for which we haven't found a greater element yet, allowing efficient lookup of the next greater element.

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: Next Greater Element means first bigger number on right [OK]

Common Mistakes:

• Confusing next greater with next smaller element
• Thinking it finds the maximum element overall
• Assuming it finds elements to the left instead of right

2. Which of the following C# code snippets correctly initializes a stack to solve the Next Greater Element problem?

easy

A. int[] stack = new int[10];

B. Stack stack = new Stack();

C. List<int> stack = new List<int>();

D. Stack<int> stack = new Stack<int>();

Solution

1. Step 1: Identify correct stack initialization syntax in C#

   Using generic Stack<int> with new Stack<int>() is the modern and type-safe way to create a stack of integers.

2. Step 2: Compare other options

   Stack stack = new Stack(); uses non-generic Stack (older style), C uses List which is not a stack, D uses array which is fixed size and not a stack.

3. Final Answer:

   Stack<int> stack = new Stack<int>(); -> Option D

4. Quick Check:

   Generic Stack<int> initialization is correct [OK]

Hint: Use generic Stack<int> for integer stacks in C# [OK]

Common Mistakes:

• Using non-generic Stack without type safety
• Using List instead of Stack for LIFO behavior
• Using fixed-size arrays instead of dynamic stack

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

medium

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

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

C. [25, 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: Form the output array

   Output array is [5, 25, 25, -1].

3. Final Answer:

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

4. Quick Check:

   Next Greater Elements = [5, 25, 25, -1] [OK]

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

Common Mistakes:

• Assigning -1 too early for elements with greater elements later
• Confusing next greater with next smaller
• Misreading the input array order

4. Identify the error in the following C# code snippet for Next Greater Element:

int[] arr = {2, 7, 3, 5, 4, 6, 8};
Stack<int> stack = new Stack<int>();
int[] result = new int[arr.Length];
for (int i = 0; i < arr.Length; i++) {
  while (stack.Count > 0 && arr[stack.Peek()] < arr[i]) {
    result[stack.Pop()] = arr[i];
  }
  stack.Push(i);
}
while (stack.Count > 0) {
  result[stack.Pop()] = 0;
}

medium

A. Using arr[i] instead of arr[stack.Peek()] in while condition

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

C. Pushing values instead of indices onto the stack

D. Not initializing the result array

Solution

1. Step 1: Check how no greater elements are marked

   The code assigns 0 to elements with no next greater element, but conventionally -1 is used to indicate none.

2. Step 2: Verify other parts of the code

   Indices are pushed, arr[stack.Peek()] is correctly used, and result array is initialized, so no errors there.

3. Final Answer:

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

4. Quick Check:

   No greater element marked as -1, not 0 [OK]

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

Common Mistakes:

• Using 0 instead of -1 for no greater element
• Pushing values instead of indices on stack
• Incorrect while loop condition

5. Given the array [13, 7, 6, 12], how would you modify the Next Greater Element algorithm to find the next greater element to the left instead of the right?

hard

A. Traverse the array from right to left and use a stack to track elements, popping greater ones.

B. Traverse the array from right to left and use a stack to track elements, popping smaller ones.

C. Traverse the array from left to right and use a stack to track elements, popping smaller ones.

D. Traverse the array from left to right and use a queue instead of a stack.

Solution

1. Step 1: Understand direction change for next greater to left

   To find next greater element on the left, we must scan from left to right, checking elements before current.

2. Step 2: Use stack to track candidates and pop smaller elements

   We push elements and pop smaller ones to find the nearest greater on the left efficiently.

3. Final Answer:

   Traverse the array from left to right and use a stack to track elements, popping smaller ones. -> Option C

4. Quick Check:

   Next greater left = left to right traversal with stack [OK]

Hint: For next greater left, scan left to right popping smaller elements [OK]

Common Mistakes:

• Scanning right to left for next greater left
• Using queue instead of stack
• Popping greater elements instead of smaller