Say you have an array for which the ith element is the price of a given stock on day i.

Design an algorithm to find the maximum profit. You may complete as many transactions as you like (ie, buy one and sell one share of the stock multiple times). However, you may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).

分析：能夠多次買賣，先記錄下買入價格，假設碰到比買入價格低的。更新買入價格，碰到比買入價格高的，進行交易，同一時候更新買入價格，也是線性復雜度

class Solution {
public:
    int maxProfit(vector<int> &prices) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int n = prices.size();
        if(n < 2)return 0;
        
        int sum = 0;
        int buy = prices[0];
        for(int i = 1; i < n; i++)
        {
            if(prices[i] < buy)buy = prices[i];
            else if(prices[i] > buy)
            {
                sum += prices[i] - buy;
                buy = prices[i];
            }
        }
        
        return sum;
    }
};

leetcode - Best Time to Buy and Sell Stock II