Buy and Sell Stocks All Variants in DSA Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal in the 'Buy and Sell Stocks' problem?

To maximize profit by choosing the best days to buy and sell stocks given their prices over time.

beginner

In the 'Buy and Sell Stocks I' variant, how many transactions are allowed?

Only one transaction is allowed: buy once and sell once.

intermediate

What is the key difference between 'Buy and Sell Stocks I' and 'Buy and Sell Stocks II'?

'Stocks I' allows only one transaction, while 'Stocks II' allows unlimited transactions.

intermediate

How does the 'Buy and Sell Stocks with Cooldown' variant affect trading?

After selling a stock, you must wait one day before buying again (cooldown period).

advanced

What is the purpose of dynamic programming in 'Buy and Sell Stocks with Transaction Fee'?

To efficiently calculate maximum profit by considering transaction fees and multiple transactions.

In 'Buy and Sell Stocks I', what is the best strategy to maximize profit?

A Find the lowest price day to buy and highest price day after it to sell

B Buy and sell every day

C Buy on the last day and sell on the first day

D Ignore prices and pick random days

You want to buy at the lowest price and sell later at the highest price to maximize profit.

In 'Buy and Sell Stocks II', how many transactions can you make?

A Only one

B None

C Exactly two

D Unlimited

You can make as many buy-sell transactions as you want to maximize profit.

What does the cooldown period mean in 'Buy and Sell Stocks with Cooldown'?

A You cannot buy and sell on the same day

B You can only buy once

C You must wait one day after selling before buying again

D You cannot sell stocks

After selling, you must wait one day before you can buy again.

Why do we use dynamic programming in 'Buy and Sell Stocks with Transaction Fee'?

A To handle transaction fees and multiple transactions efficiently

B To avoid calculating profit repeatedly

C To sort the prices

D To buy stocks randomly

Dynamic programming helps track profits considering fees and multiple buys and sells.

Which variant restricts the number of transactions to a fixed number k?

A Buy and Sell Stocks II

B Buy and Sell Stocks with k Transactions

C Buy and Sell Stocks I

D Buy and Sell Stocks with Cooldown

The variant with k transactions limits the number of buy-sell pairs to k.

Explain the differences between the main variants of the 'Buy and Sell Stocks' problem.

Think about how many transactions are allowed and any extra rules like cooldown or fees.

Describe how dynamic programming helps solve the 'Buy and Sell Stocks' problems with constraints.

Focus on how breaking the problem into smaller parts helps.

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