class Solution {

 public:

     int maxProfit(vector<int> &prices) {

         if (prices.size() == )

              return ;

         // "minimum" holds the minimum price before the ith day.

         // "max_diff" holds the maximum difference between prices[i] and prices[j]

         // where 0 <= i < j < prices.size()

         int minimum = prices[], max_diff = ;

         for (unsigned int i = ; i < prices.size(); i++) {

             if (prices[i] - minimum > max_diff) {

                 max_diff = prices[i] - minimum;

             }

             if (prices[i] < minimum) {

                 minimum = prices[i];

             }

         }

         return max_diff;

     }

 };