# 题目描述

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 (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Note that you cannot sell a stock before you buy one.

Example 1:
Input: [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 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
Explanation: In this case, no transaction is done, i.e. max profit = 0.

## 题目解法

Runtime: 828 ms, faster than 8.10% of C++ online submissions for Best Time to Buy and Sell Stock.
Memory Usage: 9.6 MB, less than 14.91% of C++ online submissions for Best Time to Buy and Sell Stock.

## 优化方法

Runtime: 8 ms, faster than 78.13% of C++ online submissions for Best Time to Buy and Sell Stock.
Memory Usage: 9.5 MB, less than 60.83% of C++ online submissions for Best Time to Buy and Sell Stock.