122. Best Time to Buy and Sell Stock II

greedy O(n) time O(1) space
只要挣钱（后一天比前一天价格高）就买进卖出

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