［抄题］：
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)
［一句话思路］：贪心算法。不要用数组i,j角标，直接用min, profit表示价格，更方便。
［画图］：
［一刷］：
min, profit都记得要初始化
［总结］：
［复杂度］：
n/1
［英文数据结构］：
［其他解法］：
［题目变变变］：
maximum subarray最大求和子数组
