Stock Span Problem Using Stack in DSA C# - Build from Scratch

Stock Span Problem Using Stack

📖 Scenario: You are working as a stock market analyst. You want to find out how many consecutive days before today the stock price was less than or equal to today's price. This helps in understanding the stock's performance trend.

🎯 Goal: Build a program that calculates the stock span for each day using a stack data structure.

📋 What You'll Learn

Create an array called prices with the exact values: 100, 80, 60, 70, 60, 75, 85

Create an empty stack called stack to store indices

Create an integer array called span of the same length as prices

Use a for loop with variable i to iterate over prices

Calculate the stock span for each day using the stack

Print the span array values separated by spaces

💡 Why This Matters

🌍 Real World

Stock span calculation helps traders and analysts understand how long a stock price has been rising or stable, which is useful for making investment decisions.

💼 Career

Understanding stack-based algorithms and array manipulations is essential for software developers working on financial software, data analysis, and algorithmic trading.

1

Create the stock prices array

Create an integer array called prices with these exact values: 100, 80, 60, 70, 60, 75, 85

DSA C#

// Create the prices array with the given values
// Your code here

Hint

Use int[] prices = { ... }; to create the array with the exact values.

2

Set up the stack and span array

Create an empty stack called stack of type Stack<int> and an integer array called span with the same length as prices

DSA C#

int[] prices = {100, 80, 60, 70, 60, 75, 85};
// Create the stack and span array below
// Your code here

Hint

Use Stack<int> stack = new Stack<int>(); and int[] span = new int[prices.Length];

3

Calculate the stock span using a for loop and stack

Use a for loop with variable i from 0 to prices.Length - 1. Inside the loop, pop from stack while it is not empty and prices[stack.Peek()] <= prices[i]. Then set span[i] to i + 1 if stack is empty, else to i - stack.Peek(). Finally, push i onto stack.

DSA C#

using System.Collections.Generic;

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

// Calculate the span for each day using a for loop and stack
// Your code here

Hint

Use a for loop and inside it use a while loop to pop from stack while top price is less or equal to current price. Then calculate span and push current index.

4

Print the stock span array

Use Console.WriteLine to print the values of the span array separated by spaces on one line.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        int[] prices = {100, 80, 60, 70, 60, 75, 85};
        Stack stack = new Stack();
        int[] span = new int[prices.Length];

        for (int i = 0; i  0 && prices[stack.Peek()] <= prices[i])
            {
                stack.Pop();
            }

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

            stack.Push(i);
        }

        // Print the span array values separated by spaces
        // Your code here
    }
}

Hint

Use Console.WriteLine(string.Join(" ", span)); to print the span array values separated by spaces.

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