Say you have an array for which the ith element is the price of a given stock on day i.
If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.
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)
可用一个循环，记录当前遇到的数的最小值，变量可初始化为Integer.MAX_VALUE,同时用maxPro 记录最大的利益差值，省去比较其他元素，注意数组为空。
