Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a stock on day i. You want to maximize profit by choosing a single day to buy and a later day to sell. Return the maximum profit you can achieve. If no profit is possible, return 0.

Example

Input: prices = [7,1,5,3,6,4] → Output: 5 Explanation: Buy on day 2 (price=1), sell on day 5 (price=6). Profit = 6 - 1 = 5.

Input: prices = [7,6,4,3,1] → Output: 0 Explanation: Prices only decrease; no transaction gives a profit.

Constraints

• 1 ≤ prices.length ≤ 10⁵
• 0 ≤ prices[i] ≤ 10⁴