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

Buy and Sell Stocks All Variants

📖 Scenario: You are helping a friend who wants to understand how to find the best time to buy and sell stocks to maximize profit. They have a list of stock prices for consecutive days and want to learn how to find the best single buy-sell pair.

🎯 Goal: Build a simple program that finds the maximum profit from buying and selling a stock once, given a list of daily prices.

📋 What You'll Learn

Create a list of integers called prices with exact values: 7, 1, 5, 3, 6, 4

Create an integer variable called maxProfit and set it to 0

Use a for loop with variable i to iterate over prices starting from index 1

Inside the loop, update minPrice and calculate maxProfit accordingly

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, a common task in finance and investing.

💼 Career

Knowing how to solve buy and sell stock problems is useful for software roles in finance, trading platforms, and data analysis.

1

Create the stock prices list

Create a list of integers called prices with these exact values: 7, 1, 5, 3, 6, 4.

DSA C#

# Your code here

Hint

Use List<int> and initialize it with the exact values inside curly braces.

2

Set up the maxProfit and minPrice variables

Create an integer variable called maxProfit and set it to 0. Create another integer variable called minPrice and set it to the first element of prices.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List prices = new List {7, 1, 5, 3, 6, 4};
        // Your code here
    }
}

Hint

Set maxProfit to zero and minPrice to the first price in the list.

3

Calculate the maximum profit using a loop

Use a for loop with variable i starting from 1 to iterate over prices. Inside the loop, update minPrice to the smaller of minPrice and prices[i]. Then update maxProfit to the larger of maxProfit and prices[i] - minPrice.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List prices = new List {7, 1, 5, 3, 6, 4};
        int maxProfit = 0;
        int minPrice = prices[0];
        // Use a for loop to update minPrice and maxProfit
        // Your code here
    }
}

Hint

Use for (int i = 1; i < prices.Count; i++) and inside update minPrice and maxProfit using if and else if.

4

Print the maximum profit

Print the value of maxProfit using Console.WriteLine(maxProfit);.

DSA C#

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List prices = new List {7, 1, 5, 3, 6, 4};
        int maxProfit = 0;
        int minPrice = prices[0];
        for (int i = 1; i  maxProfit)
            {
                maxProfit = prices[i] - minPrice;
            }
        }
        // Print maxProfit
        // Your code here
    }
}

Hint

Use Console.WriteLine(maxProfit); to show the result.

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