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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code for max profit with one transaction?

Given the prices array, what is the maximum profit from one buy and one sell?

DSA Typescript

function maxProfitOneTransaction(prices: number[]): number {
  let minPrice = Infinity;
  let maxProfit = 0;
  for (const price of prices) {
    if (price < minPrice) {
      minPrice = price;
    } else if (price - minPrice > maxProfit) {
      maxProfit = price - minPrice;
    }
  }
  return maxProfit;
}

const prices = [7,1,5,3,6,4];
console.log(maxProfitOneTransaction(prices));

A 3

B 5

C 4

D 6

💡 Hint

Track the lowest price and calculate profit by subtracting it from current price.

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

❓ Predict Output

intermediate

What is the output of max profit with unlimited transactions?

Calculate the maximum profit if you can buy and sell stocks multiple times (unlimited transactions).

DSA Typescript

function maxProfitUnlimitedTransactions(prices: number[]): number {
  let profit = 0;
  for (let i = 1; i < prices.length; i++) {
    if (prices[i] > prices[i - 1]) {
      profit += prices[i] - prices[i - 1];
    }
  }
  return profit;
}

const prices = [7,1,5,3,6,4];
console.log(maxProfitUnlimitedTransactions(prices));

A 7

B 8

C 5

D 6

💡 Hint

Add all positive differences between consecutive days.

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

❓ Predict Output

advanced

What is the output of max profit with at most two transactions?

Find the maximum profit with at most two buy-sell transactions.

DSA Typescript

function maxProfitTwoTransactions(prices: number[]): number {
  const n = prices.length;
  if (n === 0) return 0;
  const leftProfits = new Array(n).fill(0);
  let minPrice = prices[0];
  for (let i = 1; i < n; i++) {
    minPrice = Math.min(minPrice, prices[i]);
    leftProfits[i] = Math.max(leftProfits[i - 1], prices[i] - minPrice);
  }
  const rightProfits = new Array(n + 1).fill(0);
  let maxPrice = prices[n - 1];
  for (let i = n - 2; i >= 0; i--) {
    maxPrice = Math.max(maxPrice, prices[i]);
    rightProfits[i] = Math.max(rightProfits[i + 1], maxPrice - prices[i]);
  }
  let maxProfit = 0;
  for (let i = 0; i < n; i++) {
    maxProfit = Math.max(maxProfit, leftProfits[i] + rightProfits[i + 1]);
  }
  return maxProfit;
}

const prices = [3,3,5,0,0,3,1,4];
console.log(maxProfitTwoTransactions(prices));

A 7

B 4

C 5

D 6

💡 Hint

Use two passes: left to right and right to left to track profits.

Best two transactions: buy at 0 sell at 3, buy at 1 sell at 4, total profit 3 + 3 = 6.

❓ Predict Output

advanced

What is the output of max profit with cooldown period?

Calculate max profit if after selling stock you must wait one day before buying again.

DSA Typescript

function maxProfitWithCooldown(prices: number[]): number {
  const n = prices.length;
  if (n <= 1) return 0;
  let sold = 0, hold = -prices[0], rest = 0;
  for (let i = 1; i < n; i++) {
    const prevSold = sold;
    sold = hold + prices[i];
    hold = Math.max(hold, rest - prices[i]);
    rest = Math.max(rest, prevSold);
  }
  return Math.max(sold, rest);
}

const prices = [1,2,3,0,2];
console.log(maxProfitWithCooldown(prices));

A 3

B 4

C 2

D 5

💡 Hint

Track three states: sold, hold, and rest to handle cooldown.

Best transactions: buy day 0, sell day 2, cooldown day 3, buy day 4, sell day 4, total profit 3.

❓ Predict Output

expert

What is the output of max profit with transaction fee?

Calculate max profit if each transaction costs a fee.

DSA Typescript

function maxProfitWithFee(prices: number[], fee: number): number {
  const n = prices.length;
  if (n === 0) return 0;
  let cash = 0;
  let hold = -prices[0];
  for (let i = 1; i < n; i++) {
    cash = Math.max(cash, hold + prices[i] - fee);
    hold = Math.max(hold, cash - prices[i]);
  }
  return cash;
}

const prices = [1,3,2,8,4,9];
const fee = 2;
console.log(maxProfitWithFee(prices, fee));

A 5

B 7

C 6

D 8

💡 Hint

Track cash and hold states, subtract fee when selling.

Transactions: buy at 1 sell at 8 profit 7-2=5, buy at 4 sell at 9 profit 5-2=3, total 8 but fee counted twice, final profit 6.