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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Single Transaction Maximum Profit

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

DSA C

int maxProfit(int* prices, int pricesSize) {
    int minPrice = 2147483647;
    int maxProfit = 0;
    for (int i = 0; i < pricesSize; i++) {
        if (prices[i] < minPrice) {
            minPrice = prices[i];
        } else if (prices[i] - minPrice > maxProfit) {
            maxProfit = prices[i] - minPrice;
        }
    }
    return maxProfit;
}

int main() {
    int prices[] = {7,1,5,3,6,4};
    int result = maxProfit(prices, 6);
    printf("%d\n", result);
    return 0;
}

A 5

B 4

C 6

D 7

💡 Hint

Think about the lowest price before the highest profit.

The lowest price is 1 and the highest price after it is 6, so max profit is 6 - 1 = 5.

❓ Predict Output

intermediate

Output of Multiple Transactions Maximum Profit

What is the output of this C code that calculates the maximum profit from multiple buy-sell transactions (buy and sell multiple times)?

DSA C

int maxProfit(int* prices, int pricesSize) {
    int profit = 0;
    for (int i = 1; i < pricesSize; i++) {
        if (prices[i] > prices[i - 1]) {
            profit += prices[i] - prices[i - 1];
        }
    }
    return profit;
}

int main() {
    int prices[] = {7,1,5,3,6,4};
    int result = maxProfit(prices, 6);
    printf("%d\n", result);
    return 0;
}

A 4

B 5

C 6

D 7

💡 Hint

Sum all positive differences between consecutive days.

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

❓ Predict Output

advanced

Output of Maximum Profit with Cooldown

What is the output of this C code that calculates the maximum profit with cooldown (cannot buy on the day after selling)?

DSA C

int maxProfit(int* prices, int pricesSize) {
    if (pricesSize == 0) return 0;
    int sell = 0, prev_sell = 0, buy = -2147483647, prev_buy;
    for (int i = 0; i < pricesSize; i++) {
        prev_buy = buy;
        buy = (prev_sell - prices[i]) > buy ? (prev_sell - prices[i]) : buy;
        prev_sell = sell;
        sell = (prev_buy + prices[i]) > sell ? (prev_buy + prices[i]) : sell;
    }
    return sell;
}

int main() {
    int prices[] = {1,2,3,0,2};
    int result = maxProfit(prices, 5);
    printf("%d\n", result);
    return 0;
}

A 5

B 4

C 3

D 2

💡 Hint

Consider cooldown day after selling.

The best sequence is buy on day 0, sell day 1, cooldown day 2, buy day 3, sell day 4 for profit 3.

❓ Predict Output

advanced

Output of 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 maxProfit(int* prices, int pricesSize, int fee) {
    int cash = 0, hold = -prices[0];
    for (int i = 1; i < pricesSize; i++) {
        cash = cash > hold + prices[i] - fee ? cash : hold + prices[i] - fee;
        hold = hold > cash - prices[i] ? hold : cash - prices[i];
    }
    return cash;
}

int main() {
    int prices[] = {1,3,2,8,4,9};
    int result = maxProfit(prices, 6, 2);
    printf("%d\n", result);
    return 0;
}

A 6

B 8

C 5

D 7

💡 Hint

Subtract fee on each sell transaction.

Transactions: buy at 1 sell at 8 profit 5 (8-1-2), buy at 4 sell at 9 profit 3 (9-4-2), total 8.

🧠 Conceptual

expert

Minimum Number of Transactions for Maximum Profit

Given an array of stock prices, what is the minimum number of transactions needed to achieve the maximum possible profit when multiple transactions are allowed?

A Number of local minima and maxima pairs

B Number of all increasing price pairs

C Number of days minus one

D Number of price peaks only

💡 Hint

Think about buying at valleys and selling at peaks.

Each transaction corresponds to buying at a local minimum and selling at the next local maximum to maximize profit with minimum transactions.