index.md

121. Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a given stock on the ith day.

You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Example 1:

Input: prices = [7,1,5,3,6,4] Output: 5 Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5. Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Example 2:

Input: prices = [7,6,4,3,1] Output: 0 Explanation: In this case, no transactions are done and the max profit = 0.

Constraints

1 <= prices.length <= 10^5 0 <= prices[i] <= 10^4

Solution

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        min_price = float('inf')  # Initialize min_price to a large value
        max_profit = 0  

        for price in prices:
            # Update min_price if the current price is lower
            min_price = min(min_price, price)
            
            # Update max_profit if selling at the current price yields a higher profit
            max_profit = max(max_profit, price - min_price)

        return max_profit

Thoughts

Initially thought it would be O(n^2) solutions where you have two pointer pair iterating over each pair combination to get the best profit.

Time Complexity : O(n) Space Complexity : O(1)