Practice Problems - Best Time to Buy and Sell Stock

July 24, 20251 min read

Best Time to Buy and Sell Stock

Problem Statement:

Given an array prices where prices[i] is the price of a given stock on day i, find the maximum profit you can achieve by making at most one transaction.

Pseudocode:

function maxProfit(prices):
    min_price = infinity
    max_profit = 0
    for price in prices:
        if price < min_price:
            min_price = price
        elif price - min_price > max_profit:
            max_profit = price - min_price
    return max_profit

Key Details Interviewers Look For:

Single pass solution
Tracking minimum price and maximum profit
Edge case handling (e.g., empty array)

Key Identifiers:

Array of stock prices
Maximize profit
Single transaction (buy and sell once)

Implementations

Kotlin

    fun maxProfit(prices: IntArray): Int {
        var minPrice = Int.MAX_VALUE
        var maxProfit = 0
        for (price in prices) {
            val currProfit = price - minPrice
            if (price < minPrice) {
                minPrice = price
            }
            else if (currProfit > maxProfit) {
                maxProfit = currProfit
            }
        }
        return maxProfit
    }

Python

from typing import List

def maxProfit(prices: List[int]) -> int:
    min_price = float('inf')
    max_profit = 0
    for price in prices:
        curr_price = price - min_price
        if price < min_price:
            min_price = price
        elif curr_price > max_profit:
            max_profit = curr_price
    return max_profit