Best Time to Buy and Sell Stock

Problem Given prices where prices[i] is a stock's price on day i, maximize profit from one buy followed by one later sell; return 0 if no profitable trade exists.

Input / Output

• Input: int array prices. Output: max profit.

Constraints

• n up to 10^5; O(n) single pass expected.

Example

• [7,1,5,3,6,4] → 5 (buy at 1, sell at 6); [7,6,4,3,1] → 0.