If you were only permitted to complete at most one transaction (ie, buy one and sell one share

Example 1: Input: [7, 1, 5, 3, 6, 4] Output: 5

max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)

In this case, no transaction is done, i.e. max profit = 0.