### 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
``````