Leetcode 121. Best Time to Buy and Sell Stock 最佳股票售卖时(动态规划，数组，模拟)

Input: [7, 1, 5, 3, 6, 4]
Output: 5
最大收益 = 6-1 = 5 (不是7-1 = 6,因为先买后卖，7买，1买亏了6)


Input: [7, 6, 4, 3, 1]
Output: 0
最大收益为0

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int profit = 0;
        for(auto start=prices.begin();start!=prices.end();start++){
            int buy = *start;
            int sell = *std::max_element(start,prices.end());
            if((sell - buy)>profit){
                profit = sell-buy;
            }
        }
        return profit;
    }
};

class Solution{
public:
    int maxProfit(vector<int> &prices){
        int profit = 0;
        int minBuy = std::numeric_limits<int>::max();
        for(int i=0;i<prices.size();i++){
            //buy it if current price is less than minimum prices yet
            if(prices[i]<minBuy){
                minBuy = prices[i];
            }
            //sell it if current profit got optimally
            if((prices[i]-minBuy)>profit){
                profit = prices[i]-minBuy;
            }
        }
        return profit;
    }
};