附上代码：

1 class Solution { 2 public: 3     int maxProfit(vector
     
       &prices) { 4         unsigned int len = prices.size(); 5         if (len == 0) return 0; 6         // ans: holds the total profit that has been earned 7         int ans = 0; 8         for (int i = 0; i < len; ) { 9             // buy: holds price of the ith day where 10             //      i is the maximum index satisfing11             //      prices[i-1] > prices[i] and prices[i] <= prices[i+1]12             int buy = prices[i++];13             while (i < len && prices[i] < buy) {14                 buy = prices[i++];15             }16             if (i >= len) return ans;17             // sell: holds price of the ith day where18             //       i is the maximum index satisfing19             //       prices[i-1] < prices[i] and prices[i] >= prices[i+1]20             int sell = prices[i++];21             while (i < len && prices[i] > sell) {22                 sell = prices[i++];23             }24             ans += (sell - buy);25         }26         27         return ans;28     }29 };