https://leetcode-cn.com/problems/best-time-to-buy-and-sell-stock/ 
解法1:双指针
buy指针记录购买时的价格,i指针遍历数组
若当前价格小于buy价格,则更新buy价格
若当前价格-buy价格大于当前最大利润,则更新当前最大利润
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 | class Solution {public:
 int maxProfit(vector<int>& prices) {
 int n=prices.size();
 int buy=0;
 int res=0;
 for(int i=0;i<n;i++)
 {
 if(prices[i]<prices[buy])
 buy=i;
 if(prices[i]-prices[buy]>res)
 res=prices[i]-prices[buy];
 }
 return res;
 }
 };
 
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