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.
#####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)
#####Example 2:
Input: [7, 6, 4, 3, 1]
Output: 0
In this case, no transaction is done, i.e. max profit = 0.
######思路 遍历一遍数组，按如下规则用max，min记录最大最小值： 初始min赋值为正无穷大，max赋值为0 若当前值curr比min小，则给min赋值min=curr， curr比max大，则给max赋值max=curr 最终若max-min的值大于0，则返回max-min，否则返回0