Stock Span Problem Using Stack in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize an empty stack for the stock span problem.

DSA Javascript

const stack = [1];

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

Anew Stack()

B{}

Cnull

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which creates an object, not a stack.

Using null which is not a stack.

We use an empty array [] as a stack in JavaScript.

2fill in blank

medium

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

DSA Javascript

stack.[1](i);

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

Apush

Bpop

Cshift

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes elements.

Using shift or unshift which are for queues.

Use push to add an element to the top of the stack.

3fill in blank

hard

Fix the error in the condition to pop from the stack while the current price is higher than the price at the top index of the stack.

DSA Javascript

while (stack.length > 0 && prices[i] [1] prices[stack[stack.length - 1]]) {
  stack.pop();
}

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

A<

B>

C==

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator which is incorrect.

Using equality operator which does not remove smaller prices.

We pop while the current price is greater than the price at the top of the stack.

4fill in blank

hard

Fill both blanks to calculate the span for the current day correctly.

DSA Javascript

const span = stack.length === 0 ? [1] : i - stack[[2]];

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

Ai + 1

B0

Clength - 1

Dstack.length - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 instead of i+1 for empty stack case.

Using length - 1 instead of stack.length - 1 for stack top index.

If the stack is empty, the span is the current day index plus one. Otherwise, it's the difference between current index and the top index on the stack.

5fill in blank

hard

Fill all three blanks to complete the stock span function using a stack.

DSA Javascript

function calculateSpan(prices) {
  const n = prices.length;
  const span = new Array(n).fill(0);
  const stack = [];

  for (let i = 0; i < n; i++) {
    while (stack.length > 0 && prices[i] [1] prices[stack[stack.length - 1]]) {
      stack.[2]();
    }
    span[i] = stack.length === 0 ? i [3] 1 : i - stack[stack.length - 1];
    stack.push(i);
  }
  return span;
}

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

A>

Bpop

C+

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in comparison.

Using push instead of pop to remove.

Not adding 1 to i for span calculation.

The function pops from the stack while current price is greater than the price at the top index. It uses pop() to remove indices. The span is calculated by adding 1 to the index if stack is empty.

Practice

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

easy

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

B. The maximum stock price in the entire list

C. The minimum stock price in the entire list

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

Solution

1. Step 1: Understand the problem definition

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

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

   For each day, it counts days backward until a higher price is found or start is reached.

3. Final Answer:

   The number of consecutive days with price less or equal to current day -> Option A

4. Quick Check:

   Span counts consecutive lower/equal prices before today [OK]

Hint: Span counts days with prices ≤ current day price [OK]

Common Mistakes:

• Confusing span with max or min price
• Thinking span is price difference
• Ignoring consecutive days condition

2. Which of the following JavaScript code snippets correctly initializes an empty stack for solving the Stock Span Problem?

easy

A. const stack = 0;

B. const stack = {};

C. const stack = [];

D. const stack = null;

Solution

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

   In JavaScript, a stack is usually implemented as an array to use push and pop methods.

2. Step 2: Check initialization syntax

   An empty array is initialized with [] which is correct for a stack.

3. Final Answer:

   const stack = []; -> Option C

4. Quick Check:

   Stack uses array [] in JavaScript [OK]

Hint: Use array [] to create stack in JavaScript [OK]

Common Mistakes:

• Using object {} instead of array
• Assigning null or number instead of array
• Forgetting to initialize stack before use

3. What is the output of the stock span array for the prices [100, 80, 60, 70, 60, 75, 85] using the Stock Span Problem algorithm?

medium

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

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

C. [1, 1, 1, 1, 1, 1, 1]

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

Solution

1. Step 1: Calculate span for each price

   Day 0: 100 -> span=1 (no previous days)
   Day 1: 80 < 100 -> span=1
   Day 2: 60 < 80 -> span=1
   Day 3: 70 > 60 -> span=2 (includes day 2)
   Day 4: 60 < 70 -> span=1
   Day 5: 75 > 60, 70, 60 -> span=4 (days 4,3,2,1)
   Day 6: 85 > 75, 70, 60, 80 -> span=6 (days 5,3,2,1,0)

2. Step 2: Write the span array

   The spans are [1, 1, 1, 2, 1, 4, 6].

3. Final Answer:

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

4. Quick Check:

   Span array matches calculated values [OK]

Hint: Count consecutive previous days with price ≤ current [OK]

Common Mistakes:

• Counting all previous days instead of consecutive
• Mixing up greater and less conditions
• Incorrectly including days with higher prices

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

function calculateSpan(prices) {
  const stack = [];
  const span = [];
  for (let i = 0; i < prices.length; i++) {
    while (stack.length > 0 && prices[stack[stack.length - 1]] <= prices[i]) {
      stack.pop();
    }
    span[i] = stack.length === 0 ? i + 1 : i - stack[stack.length - 1];
    stack.push(i);
  }
  return span;
}

medium

A. The span calculation formula is incorrect; it should be i + stack[stack.length - 1]

B. There is no error; the code correctly calculates the stock span

C. The stack should store prices, not indices

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

Solution

1. Step 1: Analyze the while loop condition

   The loop pops indices while the price at top index is less than or equal to current price, which is correct for span calculation.

2. Step 2: Check span calculation formula

   If stack is empty, span is i+1 (all previous days), else difference between current index and top index in stack, which is correct.

3. Step 3: Verify stack usage

   Stack stores indices, which is standard to access prices array correctly.

4. Final Answer:

   There is no error; the code correctly calculates the stock span -> Option B

5. Quick Check:

   Code matches standard stock span algorithm [OK]

Hint: Stack stores indices; span formula uses index difference [OK]

Common Mistakes:

• Changing < to <= breaks correct span logic
• Storing prices instead of indices causes wrong indexing
• Incorrect span formula leads to wrong results

5. Given the stock prices array [10, 4, 5, 90, 120, 80], how would the Stock Span Problem algorithm change if we want to find the span of days with prices strictly less than the current day's price (not less than or equal)? What is the resulting span array?

hard

A. [1, 1, 2, 4, 5, 1]

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

C. [1, 1, 1, 4, 5, 1]

D. [1, 2, 3, 4, 5, 6]

Solution

1. Step 1: Understand the strict less than condition

   Change the comparison in the algorithm from ≤ to <, so only days with price strictly less count towards span.

2. Step 2: Calculate spans with strict less than

   Day 0: 10 -> span=1
   Day 1: 4 < 10 -> span=1
   Day 2: 5 > 4 -> span=2 (days 1 and 2)
   Day 3: 90 > 5,4,10 -> span=4 (days 0,1,2,3)
   Day 4: 120 > 90 -> span=5 (days 0 to 4)
   Day 5: 80 < 120 -> span=1

3. Final Answer:

   [1, 1, 2, 4, 5, 1] -> Option A

4. Quick Check:

   Strict less than span matches [1,1,2,4,5,1] [OK]

Hint: Use < instead of ≤ in comparison for strict span [OK]

Common Mistakes:

• Not changing comparison operator correctly
• Counting equal prices in span incorrectly
• Miscounting consecutive days with strict condition