# 121.Best Time to Buy and Sell Stock

**121.Best Time to Buy and Sell Stock**

难度:Easy

> 给定一个数组，它的第 i 个元素是一支给定股票第 i 天的价格。

如果你最多只允许完成一笔交易（即买入和卖出一支股票），设计一个算法来计算你所能获取的最大利润。

注意你不能在买入股票前卖出股票。

```
示例 1:

输入: [7,1,5,3,6,4]
输出: 5
解释: 在第 2 天（股票价格 = 1）的时候买入，在第 5 天（股票价格 = 6）的时候卖出，最大利润 = 6-1 = 5 。
     注意利润不能是 7-1 = 6, 因为卖出价格需要大于买入价格。
示例 2:

输入: [7,6,4,3,1]
输出: 0

解释: 在这种情况下, 没有交易完成, 所以最大利润为 0。
```

第n天的价格为前n-1天的价格与第n天减去前n-1天价格的最小值的最大值。

```
class Solution {
public:
    int maxProfit(vector<int>& prices) {
        
        int res=0;
         if(prices.size()<2) return res;
        int minimum=prices[0];
       
        for(int i=1;i<prices.size();i++)
        {
            minimum=min(minimum,prices[i]);
            res=max(res, prices[i]-minimum);
        }
        return res;
    }
};
```

> 执行用时 : 12 ms, 在Best Time to Buy and Sell Stock的C++提交中击败了94.05% 的用户\
> 内存消耗 : 9.5 MB, 在Best Time to Buy and Sell Stock的C++提交中击败了28.77% 的用户


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