## Challenge 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 to sell that stock in the future.
• Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.
• This challenge corresponds to LeetCode #121.

### Constraints

• `1 <= prices.length <= 105`
• `0 <= prices[i] <= 104`

### 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

Below is my solution and some test cases. This solution has a linear time complexity O(n) and a constant space complexity O(1), where n is the length of the input list.