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.

Problem Constraints:
1 <= prices. length <= 105
0 <= prices [i] <= 104

Sample Input 1:
[7,1,5,3,6,4]

Sample Output 1:
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.

Sample Input 2:
[7, 6, 4, 3, 1]

Sample Output 2:
0

Explanation:
In this case, no transactions are done, and the max profit = 0.