lastP记录目前为止看到的最低开仓价格
只要第i天的价格比lastP高，就获利了结

import java.util.*;

public class Solution {
    public int maxProfit (int[] prices) {
      if (prices.length <= 1) return 0;
      int lastP = Integer.MAX_VALUE;
      int profit = 0;
      
      for (int p : prices) {
        if (p > lastP) 
          profit += p - lastP;
        lastP = p;
      }
      return profit;
    }
}