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

```
class Solution:
def maxProfit(self, prices: List[int]) -> int:
profit = 0
max_profit = -10001
for i in range(0,len(prices)):
for j in range(i+1,len(prices)):
profit = prices[j]-prices[i]
if max_profit <= profit:
max_profit = profit
if max_profit < 0:
#prices.pop(0)
i+=1
if max_profit < 0:
max_profit = 0
return max_profit
```

결과 : log n^2 이중 for문 사용으로 인한 시간 초과