数组后一个减去前一个，为正数就加起来

public class Solution {
    public int maxProfit (int[] prices) {
        int sum = 0;
        for (int i = 1; i < prices.length; i++) {
            int profit = prices[i] - prices[i-1];
            if (profit > 0) {
                sum += profit;
            }
        }
        return sum;
    }
}