Problem

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:

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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:

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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

Solve

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class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        min_price = float('inf') #正無窮大
        max_profit = 0

        for price in prices:
            min_price = min(min_price, price)
            max_profit = max(max_profit, price - min_price)

        return max_profit

O (n) time and O(1) space

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class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        _min = prices[0]
        maxProfit = 0

        for i in range(len(prices)):
            _min = min(_min , prices[i])
            if prices[i]- _min   > maxProfit:
                maxProfit = prices[i]- _min
        return maxProfit

O (n) time and O(1) space