121. Best Time to Buy and Sell Stock买卖股票12

［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