### 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
```