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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of single transaction max profit calculation

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

DSA Go

package main
import "fmt"
func maxProfit(prices []int) int {
    minPrice := prices[0]
    maxProfit := 0
    for _, price := range prices {
        if price < minPrice {
            minPrice = price
        } else if profit := price - minPrice; profit > maxProfit {
            maxProfit = profit
        }
    }
    return maxProfit
}
func main() {
    prices := []int{7,1,5,3,6,4}
    fmt.Println(maxProfit(prices))
}

A 4

B 5

C 7

D 6

💡 Hint

Track the lowest price and calculate profit at each step.

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

❓ Predict Output

intermediate

Output of multiple transactions max profit calculation

What is the output of this Go code that calculates the maximum profit with unlimited transactions?

DSA Go

package main
import "fmt"
func maxProfit(prices []int) int {
    profit := 0
    for i := 1; i < len(prices); i++ {
        if prices[i] > prices[i-1] {
            profit += prices[i] - prices[i-1]
        }
    }
    return profit
}
func main() {
    prices := []int{7,1,5,3,6,4}
    fmt.Println(maxProfit(prices))
}

A 7

B 5

C 6

D 4

💡 Hint

Sum all positive differences between consecutive days.

Profits: (5-1)+(6-3) = 4+3=7 total profit.

❓ Predict Output

advanced

Output of max profit with cooldown period

What is the output of this Go code that calculates max profit with a cooldown day after selling?

DSA Go

package main
import "fmt"
func maxProfit(prices []int) int {
    n := len(prices)
    if n <= 1 {
        return 0
    }
    sell, prevSell, buy := 0, 0, -prices[0]
    for i := 1; i < n; i++ {
        prevBuy := buy
        buy = max(prevSell - prices[i], buy)
        prevSell = sell
        sell = max(prevBuy + prices[i], sell)
    }
    return sell
}
func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}
func main() {
    prices := []int{1,2,3,0,2}
    fmt.Println(maxProfit(prices))
}

A 3

B 4

C 2

D 5

💡 Hint

Use states: buy, sell, cooldown to track profits.

Max profit is 3 by buying day 0, selling day 2, cooldown day 3, buy day 4, sell day 4.

❓ Predict Output

advanced

Output of max profit with transaction fee

What is the output of this Go code that calculates max profit with a transaction fee per sell?

DSA Go

package main
import "fmt"
func maxProfit(prices []int, fee int) int {
    n := len(prices)
    cash, hold := 0, -prices[0]
    for i := 1; i < n; i++ {
        cash = max(cash, hold + prices[i] - fee)
        hold = max(hold, cash - prices[i])
    }
    return cash
}
func max(a, b int) int {
    if a > b {
        return a
    }
    return b
}
func main() {
    prices := []int{1,3,2,8,4,9}
    fee := 2
    fmt.Println(maxProfit(prices, fee))
}

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

Minimum number of transactions to reach target profit

Given a list of daily stock prices and a target profit, which algorithmic approach best finds the minimum number of buy-sell transactions needed to reach at least the target profit?

A Use a sliding window to find the longest subarray with profit equal to target.

B Use a greedy approach summing all positive price differences until target is met.

C Sort prices and pick the largest differences first until target profit is reached.

D Use dynamic programming to explore all transaction combinations and track minimum transactions for target profit.

💡 Hint

Consider all possible transactions and track minimum count for target profit.

Dynamic programming explores all combinations to find minimum transactions meeting target profit.

Practice

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

easy

A. Find the day with the highest stock price only

B. Minimize the number of transactions regardless of profit

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 goal

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

2. Step 2: Identify the correct objective

   Maximizing profit means choosing days to buy low and sell high, not just counting transactions or prices.

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 prices or counts [OK]

Common Mistakes:

• Confusing profit with number of transactions
• Thinking only about highest price day
• Ignoring the buy day in calculations

2. Which of the following Go code snippets correctly initializes a slice to store stock prices for 5 days?

easy

A. prices := []int{7, 1, 5, 3, 6}

B. prices := [5]int{7, 1, 5, 3, 6}

C. prices := make([]int, 5, 5)

D. prices := new([]int{7, 1, 5, 3, 6})

Solution

1. Step 1: Understand slice initialization in Go

   To create a slice with values, use the literal syntax: []int{values}.

2. Step 2: Check each option

   prices := []int{7, 1, 5, 3, 6} correctly creates a slice with given values. prices := [5]int{7, 1, 5, 3, 6} creates an array, not a slice. prices := make([]int, 5, 5) creates an empty slice of length 5 but no values. prices := new([]int{7, 1, 5, 3, 6}) is invalid syntax.

3. Final Answer:

   prices := []int{7, 1, 5, 3, 6} -> Option A

4. Quick Check:

   Slice literal = []int{values} [OK]

Hint: Use []int{...} to create slices with values directly [OK]

Common Mistakes:

• Using array syntax instead of slice
• Using new() incorrectly for slices
• Creating empty slices without values

3. What is the output of the following Go code that finds the max profit with only one transaction?

func maxProfit(prices []int) int {
    minPrice := prices[0]
    maxProfit := 0
    for _, price := range prices {
        if price < minPrice {
            minPrice = price
        } else if price - minPrice > maxProfit {
            maxProfit = price - minPrice
        }
    }
    return maxProfit
}

fmt.Println(maxProfit([]int{7,1,5,3,6,4}))

medium

A. 5

B. 6

C. 4

D. 7

Solution

1. Step 1: Trace the loop to find minPrice and maxProfit

   Start minPrice=7, maxProfit=0. Iterate prices: - price=7: minPrice=7, maxProfit=0 - price=1: minPrice=1 (lower) - price=5: profit=5-1=4 >0, maxProfit=4 - price=3: profit=3-1=2 <4, maxProfit=4 - price=6: profit=6-1=5 >4, maxProfit=5 - price=4: profit=4-1=3 <5, maxProfit=5

2. Step 2: Return maxProfit

   maxProfit is 5 after loop ends.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Max profit = 5 [OK]

Hint: Track min price and update max profit during iteration [OK]

Common Mistakes:

• Not updating minPrice correctly
• Calculating profit before updating minPrice
• Returning wrong variable

4. Identify the bug in this Go function that tries to calculate max profit with unlimited transactions:

func maxProfit(prices []int) int {
    profit := 0
    for i := 0; i < len(prices); i++ {
        if prices[i] > prices[i-1] {
            profit += prices[i] - prices[i-1]
        }
    }
    return profit
}

medium

A. prices slice is not initialized

B. Incorrect profit calculation logic

C. Missing return statement

D. Index out of range error due to accessing prices[i-1] when i=0

Solution

1. Step 1: Check loop and index usage

   The loop starts at i=0, but code accesses prices[i-1], which is prices[-1] at i=0, causing index out of range error.

2. Step 2: Identify fix

   Start loop from i=1 to avoid negative index access.

3. Final Answer:

   Index out of range error due to accessing prices[i-1] when i=0 -> Option D

4. Quick Check:

   Loop index must start at 1 to avoid error [OK]

Hint: Avoid negative indices by starting loop at 1 [OK]

Common Mistakes:

• Starting loop at 0 but accessing i-1
• Ignoring runtime errors
• Not testing with small input arrays

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

hard

A. 7

B. 6

C. 5

D. 8

Solution

1. Step 1: Understand two transactions limit

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

2. Step 2: Calculate max profit with two transactions

   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

3. Final Answer:

   6 -> Option B

4. Quick Check:

   Two transactions max profit = 6 [OK]

Hint: Split array, find max profit left and right, sum max [OK]

Common Mistakes:

• Counting overlapping transactions
• Ignoring zero price days
• Calculating only one transaction profit