121. Best Time to Buy and Sell Stock

Say you have an array for which the i^th 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.

我的思路：n*n的时间复杂度，从前向后，每次找出后面比当前数小的数求差，然后和最大差比较后更新最大差。

思路2：第i天卖出的收益为第i天的股价与前面i-1天中最小股价的差。从前向后遍历的过程中，记录当前最小的股价和第i天的收益。然后将第i天的收益和全局最大收益作对比以决定是否更新全局最大收益。