Stock Span Problem Using Stack in DSA Java - Build from Scratch

Stock Span Problem Using Stack

📖 Scenario: You are working as a stock 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 Java program that calculates the stock span for each day using a stack. The stock span is the count of consecutive days the price was less than or equal to the current day's price.

📋 What You'll Learn

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

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

Use a Stack<Integer> to store indices of days

Calculate the stock span for each day using the stack

Print the span array values separated by spaces

💡 Why This Matters

🌍 Real World

Stock analysts use the stock span problem to understand how long a stock price has been rising or stable, helping in making investment decisions.

💼 Career

Understanding stack-based algorithms and array manipulations is essential for software developers working on financial software and real-time data analysis.

1

Create the prices array

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

DSA Java

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

Hint

Use int[] prices = {100, 80, 60, 70, 60, 75, 85}; to create the array.

2

Create the span array and stack

Create an integer array called span of the same length as prices and create a Stack<Integer> called stack to store indices.

DSA Java

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

Hint

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

3

Calculate the stock span using the stack

Use a for loop with variable i from 0 to prices.length. 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 Java

import java.util.Stack;

int[] prices = {100, 80, 60, 70, 60, 75, 85};
int[] span = new int[prices.length];
Stack stack = new Stack();
// Calculate the stock span using the stack
// Your code here

Hint

Use a loop and stack operations as described to calculate the span.

4

Print the span array

Use a for loop with variable i from 0 to span.length and print each span[i] followed by a space on the same line.

DSA Java

import java.util.Stack;

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

for (int i = 0; i < prices.length; i++) {
    while (!stack.isEmpty() && prices[stack.peek()] <= prices[i]) {
        stack.pop();
    }
    span[i] = stack.isEmpty() ? (i + 1) : (i - stack.peek());
    stack.push(i);
}
// Print the span array values separated by spaces
// Your code here

Hint

Use a loop to print each span value followed by a space on the same line.

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 and including 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 difference between the highest and lowest stock price

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

Solution

1. Step 1: Understand the problem definition

   The Stock Span Problem finds, for each day, how many consecutive days before and including the current day had stock prices less than or equal to the current day's price.

2. Step 2: Clarify what is being counted

   It counts days backward until a higher price is found or the start is reached.

3. Final Answer:

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

4. Quick Check:

   Span counts consecutive lower/equal prices [OK]

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

Common Mistakes:

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

2. Which Java data structure is most suitable to efficiently solve the Stock Span Problem?

easy

A. Queue

B. ArrayList

C. HashMap

D. Stack

Solution

1. Step 1: Identify the need for tracking previous prices

   The problem requires checking previous days' prices to find the span efficiently.

2. Step 2: Choose a data structure that supports last-in-first-out access

   A stack allows pushing and popping prices or indices to quickly find the last higher price.

3. Final Answer:

   Stack -> Option D

4. Quick Check:

   Stack supports efficient previous day price checks [OK]

Hint: Use stack to track previous prices efficiently [OK]

Common Mistakes:

• Using ArrayList which is inefficient for this problem
• Choosing HashMap which doesn't maintain order
• Using Queue which is FIFO, not suitable here

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

medium

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

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

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

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

Solution

1. Step 1: Calculate span for each day

   Day 0: 100, no previous higher price, span=1
   Day 1: 80 ≤ 100? Yes, but previous higher price is 100, so span=1
   Day 2: 60 ≤ 80? Yes, but previous higher price is 80, so span=1
   Day 3: 70 > 60, span=2 (days 2 and 3)
   Day 4: 60 ≤ 70? Yes, but previous higher price is 70, so span=1
   Day 5: 75 > 60 and 70, span=4 (days 2,3,4,5)
   Day 6: 85 > 75, 70, 60, span=6 (days 1 to 6)

2. Step 2: Confirm final 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 lower/equal days [OK]

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

Common Mistakes:

• Counting all previous days instead of consecutive
• Misapplying stack logic causing wrong spans
• Confusing span with price values

4. Identify the error in this Java snippet for Stock Span calculation:

for (int i = 0; i < prices.length; i++) {
  while (!stack.isEmpty() && prices[stack.peek()] < prices[i]) {
    stack.pop();
  }
  span[i] = stack.isEmpty() ? i + 1 : i - stack.peek();
  stack.push(i);
}

medium

A. The stack should store prices, not indices

B. The span calculation should use i + stack.peek() instead of i - stack.peek()

C. The comparison operator should be <= instead of <

D. The loop should start from i = 1 instead of 0

Solution

1. Step 1: Analyze the while condition for popping stack

   The condition uses < to pop indices with prices less than current price, but it should pop prices less than or equal to current price to correctly count span.

2. Step 2: Confirm correct comparison operator

   Using <= ensures all previous days with price ≤ current are popped, giving correct span.

3. Final Answer:

   The comparison operator should be <= instead of < -> Option C

4. Quick Check:

   Use <= to pop all lower or equal prices [OK]

Hint: Use <= in while to pop all lower or equal prices [OK]

Common Mistakes:

• Using < instead of <= causing wrong spans
• Storing prices instead of indices in stack
• Starting loop at wrong index

5. Given stock prices [10, 4, 5, 90, 120, 80], what is the span array if the stack is implemented to store prices instead of indices?

hard

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

B. [1, 1, 1, 1, 1, 1]

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

D. [1, 1, 2, 1, 2, 1]

Solution

1. Step 1: Understand stack storing prices instead of indices

   Without indices, we cannot calculate span as difference of indices; span calculation fails.

2. Step 2: Resulting span values

   Each day is treated as isolated, so span defaults to 1 for all days.

3. Final Answer:

   [1, 1, 1, 1, 1, 1] -> Option B

4. Quick Check:

   Stack must store indices for correct span [OK]

Hint: Stack must store indices, not prices, for correct span [OK]

Common Mistakes:

• Storing prices causes incorrect span calculation
• Assuming span grows without index info
• Ignoring index difference in span formula