Problem statement

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.

Solution

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        """
            Constraints:
            - 1 <= prices.length <= 10^5
            - 0 <= prices[i] <= 10^4
            Time: O(N)
            Space: O(1)
        """
        min_val = 10_001
        ans = 0
        """
            prices =    7   1   5   3   6   4
                                            ^i
            min_price = 1
            ans = 5
        """

        # Find the largest spread between the 
        # previous min price and current price.

        for i, price in enumerate(prices):
            ans = max(ans, price - min_price)
            min_price = min(min_price, price)

        return ans