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 <= 105
0 <= prices[i] <= 104
```

한 번의 거래로 낼 수 있는 최대 이익을 산출하는 문제.

```
class Solution:
def maxProfit(self, prices: List[int]) -> int:
profit = 0
min_price = sys.maxsize
for price in prices:
min_price = min(min_price, price)
profit = max(profit, price-min_price)
return profit
```