Buy and Sell Stocks All Variants in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the maximum profit from one transaction.

DSA Javascript

function maxProfit(prices) {
  let minPrice = Infinity;
  let maxProfit = 0;
  for (let i = 0; i < prices.length; i++) {
    if (prices[i] < [1]) {
      minPrice = prices[i];
    } else if (prices[i] - minPrice > maxProfit) {
      maxProfit = prices[i] - minPrice;
    }
  }
  return maxProfit;
}

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

AminPrice

BmaxProfit

Cprices[i]

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using maxProfit instead of minPrice to compare prices.

Not updating minPrice correctly.

Comparing with 0 instead of minPrice.

We update minPrice when we find a price lower than the current minPrice to track the lowest buying price.

2fill in blank

medium

Complete the code to calculate max profit with unlimited transactions.

DSA Javascript

function maxProfit(prices) {
  let profit = 0;
  for (let i = 1; i < prices.length; i++) {
    if (prices[i] > [1]) {
      profit += prices[i] - prices[i - 1];
    }
  }
  return profit;
}

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

Aprices[i]

Bprices[i - 1]

Cprofit

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing prices[i] with prices[i] instead of prices[i - 1].

Adding profit when prices[i] is not greater than prices[i - 1].

We buy at prices[i - 1] and sell at prices[i] whenever prices[i] is higher to accumulate profit.

3fill in blank

hard

Fix the error in the code to calculate max profit with at most two transactions.

DSA Javascript

function maxProfit(prices) {
  let firstBuy = Infinity, firstProfit = 0;
  let secondBuy = Infinity, secondProfit = 0;
  for (let price of prices) {
    firstBuy = Math.min(firstBuy, price);
    firstProfit = Math.max(firstProfit, price - firstBuy);
    secondBuy = Math.min(secondBuy, price - [1]);
    secondProfit = Math.max(secondProfit, price - secondBuy);
  }
  return secondProfit;
}

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

AfirstProfit

BsecondProfit

CfirstBuy

DsecondBuy

Attempts:

3 left

💡 Hint

Common Mistakes

Using firstBuy instead of firstProfit in the subtraction.

Using secondProfit or secondBuy incorrectly.

We subtract firstProfit from price to simulate reinvesting the profit from the first transaction.

4fill in blank

hard

Fill both blanks to implement max profit with cooldown period.

DSA Javascript

function maxProfit(prices) {
  let sell = 0, prevSell = 0, buy = -Infinity;
  for (let price of prices) {
    let prevBuy = buy;
    buy = Math.max(buy, prevSell - [1]);
    prevSell = sell;
    sell = Math.max(sell, prevBuy + [2]);
  }
  return sell;
}

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

Aprice

B1

C0

D-price

Attempts:

3 left

💡 Hint

Common Mistakes

Using positive price when buying instead of negative.

Mixing up the order of updates for buy and sell.

We subtract price when buying and add price when selling to track profits correctly with cooldown.

5fill in blank

hard

Fill all three blanks to implement max profit with transaction fee.

DSA Javascript

function maxProfit(prices, fee) {
  let cash = 0, hold = -prices[0];
  for (let i = 1; i < prices.length; i++) {
    cash = Math.max(cash, hold + [1] - [2]);
    hold = Math.max(hold, cash - [3]);
  }
  return cash;
}

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

Aprices[i]

Bfee

Cprices[i - 1]

Dprices[i + 1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong price index like i-1 or i+1.

Not subtracting fee when selling.

When selling, subtract fee from prices[i]. When buying, subtract prices[i].

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