Buy and Sell Stocks All Variants in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Maximum Profit with One Transaction

What is the output of the following C# code that calculates the maximum profit from a single buy-sell transaction?

DSA C#

int[] prices = {7, 1, 5, 3, 6, 4};
int minPrice = int.MaxValue;
int maxProfit = 0;
foreach (int price in prices) {
    if (price < minPrice) minPrice = price;
    else if (price - minPrice > maxProfit) maxProfit = price - minPrice;
}
Console.WriteLine(maxProfit);

A 6

B 5

C 4

D 7

💡 Hint

Track the lowest price so far and calculate profit at each step.

The lowest price is 1, and the highest profit is 6 - 1 = 5.

❓ Predict Output

intermediate

Maximum Profit with Unlimited Transactions

What is the output of this C# code that calculates the maximum profit from unlimited buy-sell transactions?

DSA C#

int[] prices = {7, 1, 5, 3, 6, 4};
int maxProfit = 0;
for (int i = 1; i < prices.Length; i++) {
    if (prices[i] > prices[i - 1]) {
        maxProfit += prices[i] - prices[i - 1];
    }
}
Console.WriteLine(maxProfit);

A 7

B 5

C 6

D 4

💡 Hint

Add all positive differences between consecutive days.

Profit is (5-1)+(6-3) = 4 + 3 = 7.

❓ Predict Output

advanced

Maximum Profit with Cooldown Period

What is the output of this C# code that calculates the maximum profit with a cooldown period after selling?

DSA C#

int[] prices = {1, 2, 3, 0, 2};
int n = prices.Length;
int[] sell = new int[n];
int[] buy = new int[n];
buy[0] = -prices[0];
sell[0] = 0;
for (int i = 1; i < n; i++) {
    buy[i] = Math.Max(buy[i - 1], (i - 2 >= 0 ? sell[i - 2] : 0) - prices[i]);
    sell[i] = Math.Max(sell[i - 1], buy[i - 1] + prices[i]);
}
Console.WriteLine(sell[n - 1]);

A 2

B 4

C 5

D 3

💡 Hint

Use dynamic programming arrays for buy and sell states with cooldown.

Maximum profit is 3 by buying on day 0 (price 1), selling on day 1 (price 2, profit 1), cooldown on day 2, buying on day 3 (price 0), selling on day 4 (price 2, profit 2).

❓ Predict Output

advanced

Maximum Profit with Transaction Fee

What is the output of this C# code that calculates the maximum profit with a transaction fee for each sell?

DSA C#

int[] prices = {1, 3, 2, 8, 4, 9};
int fee = 2;
int n = prices.Length;
int cash = 0, hold = -prices[0];
for (int i = 1; i < n; i++) {
    cash = Math.Max(cash, hold + prices[i] - fee);
    hold = Math.Max(hold, cash - prices[i]);
}
Console.WriteLine(cash);

A 7

B 6

C 8

D 5

💡 Hint

Track cash and hold states considering fee on sell.

Max profit is 8 after accounting for fees on transactions.

🧠 Conceptual

expert

Understanding State Transitions in Buy and Sell Stocks with Cooldown

In the buy and sell stocks problem with cooldown, which statement correctly describes the state transition for the 'buy' state on day i?

A buy[i] = max(buy[i-1], sell[i-2] - price[i]) meaning you either keep holding or buy today after cooldown

B buy[i] = max(buy[i-1], sell[i-1] - price[i]) meaning you can buy immediately after selling yesterday

C buy[i] = max(sell[i-1], buy[i-1] - price[i]) meaning you buy multiple times without cooldown

D buy[i] = sell[i-2] + price[i] meaning you must sell two days before buying

💡 Hint

Cooldown means you cannot buy immediately after selling; you must wait one day.

You can buy today only if you rested yesterday (sell[i-2]) or keep holding from before (buy[i-1]).

Practice

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

easy

A. Minimize the number of transactions regardless of profit

B. Find the stock price on the last day only

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

D. Calculate the average stock price over all days

Solution

1. Step 1: Understand the problem objective

   The problem asks to maximize profit by buying and selling stocks on certain days.

2. Step 2: Identify the key action

   Choosing the best days to buy low and sell high maximizes profit.

3. Final Answer:

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

4. Quick Check:

   Goal = Maximize profit [OK]

Hint: Focus on profit maximization, not just transactions [OK]

Common Mistakes:

• Confusing profit with number of transactions
• Ignoring the buy before sell rule
• Trying to find average price instead of max profit

2. Which of the following C# code snippets correctly initializes an array of stock prices for the Buy and Sell Stocks problem?

easy

A. int prices[] = new int[6] {7, 1, 5, 3, 6, 4};

B. int prices = {7, 1, 5, 3, 6, 4};

C. int[] prices = (7, 1, 5, 3, 6, 4);

D. int[] prices = new int[] {7, 1, 5, 3, 6, 4};

Solution

1. Step 1: Check correct array initialization syntax in C#

   In C#, arrays are initialized with int[] prices = new int[] { ... };.

2. Step 2: Verify each option

   int[] prices = new int[] {7, 1, 5, 3, 6, 4}; uses correct syntax. The other options have invalid syntax: one declares a single int without array type, another uses parentheses instead of braces, and the last has incorrect array declaration style.

3. Final Answer:

   int[] prices = new int[] {7, 1, 5, 3, 6, 4}; -> Option D

4. Quick Check:

   Correct C# array syntax = int[] prices = new int[] {7, 1, 5, 3, 6, 4}; [OK]

Hint: Use int[] arr = new int[] { ... }; for arrays [OK]

Common Mistakes:

• Omitting 'new int[]' when initializing arrays
• Using parentheses instead of braces
• Declaring arrays like in C or Java incorrectly

3. What is the output of the following C# code that finds the maximum profit with only one transaction allowed?

int[] prices = {7, 1, 5, 3, 6, 4};
int minPrice = int.MaxValue;
int maxProfit = 0;
foreach (int price in prices) {
    if (price < minPrice) minPrice = price;
    else if (price - minPrice > maxProfit) maxProfit = price - minPrice;
}
Console.WriteLine(maxProfit);

medium

A. 6

B. 5

C. 4

D. 7

Solution

1. Step 1: Trace the minimum price and max profit updates

   Prices: 7,1,5,3,6,4. Min price updates to 7 then 1. Max profit updates when price - minPrice > maxProfit: 5-1=4, 6-1=5.

2. Step 2: Identify final max profit

   Max profit is 5, achieved by buying at 1 and selling at 6.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Max profit one transaction = 5 [OK]

Hint: Track min price and update max profit on each day [OK]

Common Mistakes:

• Confusing max profit with max price
• Not updating min price correctly
• Calculating profit without checking min price

4. The following code is intended to calculate the maximum profit with unlimited transactions allowed. What is the bug?

int maxProfit = 0;
for (int i = 1; i < prices.Length; i++) {
    if (prices[i] > prices[i - 1]) {
        maxProfit += prices[i] - prices[i - 1];
    }
}
return maxProfit;

medium

A. There is no bug; code works correctly

B. The code does not handle empty price array

C. The loop should start from i = 0

D. The code incorrectly adds profit when prices decrease

Solution

1. Step 1: Understand the logic for unlimited transactions

   The code adds profit for every increase from day i-1 to i, which is correct for unlimited transactions.

2. Step 2: Check for bugs

   The loop starts at 1 to compare with previous day. It skips empty array handling but not asked here. No incorrect profit addition.

3. Final Answer:

   There is no bug; code works correctly -> Option A

4. Quick Check:

   Unlimited transactions profit logic = correct [OK]

Hint: Add profit only when price increases day-to-day [OK]

Common Mistakes:

• Starting loop at wrong index
• Adding profit on price drops
• Ignoring empty or single-element arrays

5. Given the prices array {3, 3, 5, 0, 0, 3, 1, 4} and a limit of at most two transactions, what is the maximum profit achievable?

hard

A. 8

B. 7

C. 5

D. 6

Solution

1. Step 1: Understand the two transactions limit

   We can buy and sell at most twice to maximize profit.

2. Step 2: Calculate max profit by splitting days

   First transaction: buy at 3, sell at 5 (profit 2). Second transaction: buy at 0, sell at 4 (profit 4). Total profit = 2 + 4 = 6. But better transactions are buy at 0 sell at 3 (profit 3), then buy at 1 sell at 4 (profit 3), total 6. However, the maximum profit is actually 8 by buying at 0 and selling at 5 (profit 5), then buying at 1 and selling at 4 (profit 3).

3. Final Answer:

   8 -> Option A

4. Quick Check:

   Two transactions max profit = 8 [OK]

Hint: Split array to find best two transaction profits [OK]

Common Mistakes:

• Counting more than two transactions
• Not choosing optimal buy/sell days
• Adding profits incorrectly