1 class Solution {
 2 public:
 3     int maxProfit(vector<int> &prices) {
 4         if (prices.size() == 0) 
 5              return 0;
 6         // "minimum" holds the minimum price before the ith day.
 7         // "max_diff" holds the maximum difference between prices[i] and prices[j]
 8         // where 0 <= i < j < prices.size()
 9         int minimum = prices[0], max_diff = 0;
10         for (unsigned int i = 1; i < prices.size(); i++) {
11             if (prices[i] - minimum > max_diff) {
12                 max_diff = prices[i] - minimum;
13             }
14             if (prices[i] < minimum) {
15                 minimum = prices[i];
16             }
17         }
18         return max_diff;
19     }
20 };