Stock Span Problem Using Stack in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a stack of integers for storing indices.

DSA C#

Stack<int> stack = new Stack<int>[1];

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

A<>

B()

C[]

D{}

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] instead of parentheses for constructor.

Using curly braces {} which are for collection initializers.

In C#, to create a new instance of a generic Stack, you use parentheses ().

2fill in blank

medium

Complete the code to push the current index i onto the stack.

DSA C#

stack.[1](i);

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Aadd

Bappend

Cinsert

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using add or append which are not stack methods.

Using insert which is for lists.

The method to add an element to the top of a stack in C# is Push().

3fill in blank

hard

Fix the error in the while loop condition to pop indices while the current price is higher.

DSA C#

while (stack.Count > 0 && prices[i] [1] prices[stack.Peek()])
{
    stack.Pop();
}

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

A>

B<

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < or <= which reverses the logic.

Using >= which includes equal prices incorrectly.

We pop while current price is greater than price at top index to find previous higher price.

4fill in blank

hard

Fill both blanks to calculate the span correctly using stack top index.

DSA C#

span[i] = stack.Count == 0 ? i [1] 1 : i [2] stack.Peek();

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A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition when stack is empty.

Using multiplication or division which are incorrect here.

If stack is empty, span is i+1; else span is difference between current and top index.

5fill in blank

hard

Fill all three blanks to complete the stock span function returning the span array.

DSA C#

public int[] CalculateSpan(int[] prices)
{
    int n = prices.Length;
    int[] span = new int[n];
    Stack<int> stack = new Stack<int>[1];

    for (int i = 0; i < n; i++)
    {
        while (stack.Count > 0 && prices[i] [2] prices[stack.Peek()])
            stack.Pop();

        span[i] = stack.Count == 0 ? i [3] 1 : i - stack.Peek();
        stack.Push(i);
    }
    return span;
}

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

A()

B>

C+

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting parentheses in stack constructor.

Using < instead of > in while condition.

Using - instead of + when stack is empty.

Constructor uses (); condition uses >; span adds 1 when stack empty.

Practice

1. What does the Stock Span Problem calculate for each day in a list of stock prices?

easy

A. The maximum stock price in the entire list

B. The difference between the current day's price and the previous day's price

C. The number of consecutive days before the current day with a price less than or equal to the current day's price

D. The total sum of all stock prices up to the current day

Solution

1. Step 1: Understand the problem definition

   The Stock Span Problem finds how many days before today had prices less than or equal to today's price consecutively.

2. Step 2: Clarify what is calculated for each day

   For each day, it counts consecutive previous days with prices not higher than the current day.

3. Final Answer:

   The number of consecutive days before the current day with a price less than or equal to the current day's price -> Option C

4. Quick Check:

   Stock span = consecutive days with price ≤ current day price [OK]

Hint: Span counts consecutive previous days with price ≤ today [OK]

Common Mistakes:

• Confusing span with maximum price
• Counting all previous days instead of consecutive
• Calculating price differences instead of counts

2. Which of the following C# code snippets correctly declares a stack to be used for the Stock Span Problem?

easy

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

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

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

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

Solution

1. Step 1: Identify the correct data structure for stack

   In C#, Stack<T> is the built-in class for stack operations.

2. Step 2: Check each option's data structure

   Stack<int> stack = new Stack<int>(); uses Stack<int>, which is correct. int[] stack = new int[10]; uses array, List<int> stack = new List<int>(); uses List, which are not stacks. Queue<int> stack = new Queue<int>(); uses Queue, which is FIFO, not LIFO.

3. Final Answer:

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

4. Quick Check:

   Stack<T> is the correct stack type in C# [OK]

Hint: Use Stack<int> for stack, not List or Queue [OK]

Common Mistakes:

• Using List instead of Stack
• Using Queue which is FIFO, not stack
• Using fixed-size array without stack methods

3. What is the output of the following C# code snippet for stock prices [100, 80, 60, 70, 60, 75, 85]?

int[] prices = {100, 80, 60, 70, 60, 75, 85};
int[] span = new int[prices.Length];
Stack<int> stack = new Stack<int>();

for (int i = 0; i < prices.Length; i++) {
    while (stack.Count > 0 && prices[stack.Peek()] <= prices[i]) {
        stack.Pop();
    }
    span[i] = (stack.Count == 0) ? (i + 1) : (i - stack.Peek());
    stack.Push(i);
}
Console.WriteLine(string.Join(", ", span));

medium

A. [1, 1, 1, 2, 1, 4, 6]

B. [1, 2, 3, 4, 5, 6, 7]

C. [7, 6, 5, 4, 3, 2, 1]

D. [1, 1, 2, 2, 3, 3, 4]

Solution

1. Step 1: Trace the stack and span calculation for each price

   Day 0: stack empty, span[0] = 1, push 0
   Day 1: prices[0]=100 > 80, no pop, span[1] = 1 - 0 = 1, push 1
   Day 2: prices[1]=80 > 60, no pop, span[2] = 2 - 1 = 1, push 2
   Day 3: prices[2]=60 <= 70, pop 2; prices[1]=80 > 70, span[3] = 3 - 1 = 2, push 3
   Day 4: prices[3]=70 > 60, no pop, span[4] = 4 - 3 = 1, push 4
   Day 5: prices[4]=60 <= 75 pop 4; prices[3]=70 <= 75 pop 3; prices[1]=80 > 75 no pop, span[5] = 5 - 1 = 4, push 5
   Day 6: prices[5]=75 <= 85 pop 5; prices[1]=80 <= 85 pop 1; prices[0]=100 > 85 no pop, span[6] = 6 - 0 = 6, push 6

2. Step 2: Collect the span array

   The span array is [1, 1, 1, 2, 1, 4, 6].

3. Final Answer:

   [1, 1, 1, 2, 1, 4, 6] -> Option A

4. Quick Check:

   Span matches consecutive days with price ≤ current day [OK]

Hint: Pop smaller prices, span = current index - top stack index [OK]

Common Mistakes:

• Not popping all smaller or equal prices
• Using wrong index difference for span
• Confusing span with price values

4. Identify the error in the following C# code snippet for the Stock Span Problem:

int[] prices = {10, 4, 5, 90, 120, 80};
int[] span = new int[prices.Length];
Stack<int> stack = new Stack<int>();

for (int i = 0; i < prices.Length; i++) {
    while (stack.Count > 0 && prices[stack.Peek()] <= prices[i]) {
        stack.Pop();
    }
    span[i] = (stack.Count == 0) ? i : (i - stack.Peek());
    stack.Push(i);
}
Console.WriteLine(string.Join(", ", span));

medium

A. The loop should start from i = 1, not 0

B. The comparison operator in while should be <= instead of <

C. Stack should store prices, not indices

D. The span calculation should use i + 1 when stack is empty

Solution

1. Step 1: Check the span calculation when stack is empty

   When stack is empty, span should be i + 1 because all previous days are counted.

2. Step 2: Identify the error in span assignment

   Code uses span[i] = i instead of i + 1, which undercounts span for first element and others when stack is empty.

3. Final Answer:

   The span calculation should use i + 1 when stack is empty -> Option D

4. Quick Check:

   Empty stack span = i + 1, not i [OK]

Hint: Use i+1 for span if stack empty, not i [OK]

Common Mistakes:

• Using i instead of i+1 for empty stack span
• Storing prices in stack instead of indices
• Wrong comparison operator causing incorrect pops

5. Given a list of daily stock prices, how would you modify the Stock Span Problem algorithm to find the span of days where the price was strictly less than the current day's price (not less than or equal)?

hard

A. Calculate span by counting all previous days regardless of price

B. Change the comparison in the while loop from prices[stack.Peek()] <= prices[i] to prices[stack.Peek()] < prices[i]

C. Use a queue instead of a stack to track indices

D. Change the comparison in the while loop from prices[stack.Peek()] <= prices[i] to prices[stack.Peek()] >= prices[i]

Solution

1. Step 1: Understand the original comparison

   The original algorithm pops indices while prices at stack top are less than or equal to current price.

2. Step 2: Modify comparison for strictly less

   To count only days with price strictly less, change <= to < in the while condition.

3. Final Answer:

   Change the comparison in the while loop from prices[stack.Peek()] <= prices[i] to prices[stack.Peek()] < prices[i] -> Option B

4. Quick Check:

   Use < instead of ≤ for strict less span [OK]

Hint: Use < instead of <= in while condition for strict less [OK]

Common Mistakes:

• Using >= instead of < causing wrong pops
• Switching to queue which breaks logic
• Ignoring price comparison and counting all days