1 class Solution {
 2 public:
 3     int maxProfit(vector<int>& prices) {
 4         int minPrice = INT_MAX;
 5         int maxProfit = 0;
 6         for (auto ele : prices) {
 7             minPrice = min(ele, minPrice);
 8             maxProfit = max(maxProfit, (ele - minPrice));
 9         }
10         return maxProfit;
11     }
12 };

 1 class Solution {
 2 public:
 3     int maxProfit(vector<int>& prices) {
 4         const int n = prices.size();
 5         if (n < 2) {return 0;}
 6         int buy = prices[0], sell = 0;
 7         int ret = 0;
 8         for (int i = 1; i < n; ++i) {
 9             sell = prices[i];
10             if (buy < sell) {
11                 ret = max(sell - buy, ret);
12             } else {
13                 buy = prices[i];
14             }
15         }
16         return ret;
17     }
18 };

 1 class Solution {
 2 public:
 3     int maxProfit(vector<int>& prices) {
 4         const int n = prices.size();
 5         int ret = 0;
 6         for (int i = 1; i < n; ++i) {
 7             if (prices[i] - prices[i-1] > 0) {
 8                 ret += prices[i] - prices[i-1];
 9             }
10         }
11         return ret;
12     }
13 };

 1 class Solution {
 2 public:
 3     int maxProfit(vector<int>& prices) {
 4         const int n = prices.size();
 5         if (n == 0) {return 0;}
 6         vector<int> g(4, INT_MIN);
 7         g[0] = -prices[0];
 8         for (int i = 1; i < n; ++i) {
 9             g[0] = max(g[0], -prices[i]);
10             g[1] = max(g[1], g[0]+prices[i]);
11             g[2] = max(g[2], g[1]-prices[i]);
12             g[3] = max(g[3], g[2]+prices[i]);
13         }
14         return max(0, max(g[1], g[3]));
15     }
16 };