Buy and Sell Stocks All Variants in DSA Javascript - Build from Scratch

Buy and Sell Stocks All Variants

📖 Scenario: You are helping a friend who wants to understand how to make money by buying and selling stocks. They have a list of stock prices for several days and want to find the best way to buy and sell to maximize profit.

🎯 Goal: Build a simple program that finds the best day to buy and sell stocks to get the maximum profit. You will start by creating the stock prices list, then set up a variable to track profit, write the logic to find the best profit, and finally print the result.

📋 What You'll Learn

Create an array called prices with the exact values: [7, 1, 5, 3, 6, 4]

Create a variable called maxProfit and set it to 0

Use a for loop with variable i to iterate over prices

Inside the loop, use another for loop with variable j to check all days after i

Calculate profit as prices[j] - prices[i] and update maxProfit if this profit is higher

Print the value of maxProfit

💡 Why This Matters

🌍 Real World

This project helps understand how to analyze stock prices to make smart buy and sell decisions for profit.

💼 Career

Knowing how to work with arrays and loops to solve problems is essential for software development and data analysis jobs.

1

Create the stock prices array

Create an array called prices with these exact values: [7, 1, 5, 3, 6, 4]

DSA Javascript

# Your code here

Hint

Use const to create the array named prices with the exact numbers inside square brackets.

2

Set up the maxProfit variable

Create a variable called maxProfit and set it to 0

DSA Javascript

const prices = [7, 1, 5, 3, 6, 4];
# Your code here

Hint

Use let to create a variable named maxProfit and set it to zero.

3

Find the maximum profit with nested loops

Use a for loop with variable i to iterate over prices. Inside it, use another for loop with variable j to check all days after i. Calculate profit as prices[j] - prices[i] and update maxProfit if this profit is higher.

DSA Javascript

const prices = [7, 1, 5, 3, 6, 4];
let maxProfit = 0;
# Your code here

Hint

Use two loops: the outer loop with i from 0 to prices.length, and the inner loop with j from i + 1 to prices.length. Calculate profit and update maxProfit if profit is bigger.

4

Print the maximum profit

Write console.log(maxProfit) to display the maximum profit found.

DSA Javascript

const prices = [7, 1, 5, 3, 6, 4];
let maxProfit = 0;
for (let i = 0; i  maxProfit) {
            maxProfit = profit;
        }
    }
}
# Your code here

Hint

Use console.log(maxProfit) to print the final answer.

Practice

1. What is the main goal when solving the Buy and Sell Stocks problem variants?

easy

A. Maximize the profit by choosing the best days to buy and sell stocks

B. Minimize the number of transactions regardless of profit

C. Find the day with the highest stock price only

D. Calculate the average stock price over all days

Solution

1. Step 1: Understand the problem goal

   The problem asks to find the best days to buy and sell stocks to get the highest profit.

2. Step 2: Identify the objective

   Maximizing profit means buying low and selling high at the right times.

3. Final Answer:

   Maximize the profit by choosing the best days to buy and sell stocks -> Option A

4. Quick Check:

   Goal = Maximize profit [OK]

Hint: Focus on profit maximization, not just prices or counts [OK]

Common Mistakes:

• Thinking the goal is to minimize transactions
• Only looking for highest price day
• Calculating average price instead of profit

2. Which of the following is the correct syntax to initialize a variable maxProfit to 0 in JavaScript?

easy

A. var maxProfit == 0;

B. let maxProfit = 0;

C. maxProfit := 0;

D. int maxProfit = 0;

Solution

1. Step 1: Identify correct JavaScript variable declaration

   JavaScript uses let or const for variable declaration and single equals = for assignment.

2. Step 2: Check each option

   let maxProfit = 0; uses let maxProfit = 0; which is correct syntax.

3. Final Answer:

   let maxProfit = 0; -> Option B

4. Quick Check:

   Variable declaration with let and = assignment [OK]

Hint: Use let or const with single = for assignment in JavaScript [OK]

Common Mistakes:

• Using double equals == for assignment
• Using := which is not JavaScript syntax
• Using int which is not a JavaScript keyword

3. What will be the output of the following code snippet?

const prices = [7,1,5,3,6,4];
let minPrice = Infinity;
let maxProfit = 0;
for (let price of prices) {
  if (price < minPrice) minPrice = price;
  else if (price - minPrice > maxProfit) maxProfit = price - minPrice;
}
console.log(maxProfit);

medium

A. 7

B. 6

C. 4

D. 5

Solution

1. Step 1: Track minimum price and maximum profit

   Iterate prices: minPrice updates to 7, then 1; maxProfit updates when price - minPrice is larger.

2. Step 2: Calculate maxProfit step-by-step

   Prices: 7 (minPrice=7), 1 (minPrice=1), 5 (profit=4), 3 (profit=2), 6 (profit=5), 4 (profit=3). Max profit is 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Max profit = 5 [OK]

Hint: Track min price and update max profit on the fly [OK]

Common Mistakes:

• Confusing minPrice update logic
• Calculating profit incorrectly
• Using max price instead of max profit

4. The following code is intended to find the maximum profit from one transaction but has a bug. What is the bug?

function maxProfit(prices) {
  let minPrice = 0;
  let maxProfit = 0;
  for (let price of prices) {
    if (price < minPrice) minPrice = price;
    else if (price - minPrice > maxProfit) maxProfit = price - minPrice;
  }
  return maxProfit;
}

medium

A. minPrice should be initialized to Infinity, not 0

B. maxProfit should be initialized to -Infinity

C. The for loop should use index instead of for-of

D. The return statement is missing

Solution

1. Step 1: Check initialization of minPrice

   minPrice is set to 0, but prices can be greater than 0, so minPrice never updates correctly if all prices are above 0.

2. Step 2: Correct initialization

   minPrice should start at Infinity to ensure it updates to the first price correctly.

3. Final Answer:

   minPrice should be initialized to Infinity, not 0 -> Option A

4. Quick Check:

   minPrice init = Infinity [OK]

Hint: Initialize minPrice to Infinity to track minimum correctly [OK]

Common Mistakes:

• Initializing minPrice to 0 causes wrong min tracking
• Thinking maxProfit needs -Infinity initialization
• Confusing loop types for this problem

5. Given the prices array [3,3,5,0,0,3,1,4], what is the maximum profit with at most two transactions?

hard

A. 5

B. 6

C. 7

D. 4

Solution

1. Step 1: Understand two transactions limit

   We can buy and sell twice to maximize profit, but transactions cannot overlap.

2. Step 2: Calculate profits for two transactions

   First transaction: buy at 3, sell at 5 (profit 2). Second transaction: buy at 0, sell at 4 (profit 5). Total profit = 2 + 5 = 7.

3. Final Answer:

   7 -> Option C

4. Quick Check:

   Two transactions profit = 7 [OK]

Hint: Split array, find max profit in two parts, sum them [OK]

Common Mistakes:

• Trying to do more than two transactions
• Adding profits from overlapping transactions
• Missing the best buy/sell days for each transaction