Add problem 123 .

1 files changed, 34 insertions, 0 deletions

diff --git a/cpp/123.cpp b/cpp/123.cpp
new file mode 100644
index 0000000..f4feff2
--- /dev/null
+++ b/cpp/123.cpp
@@ -0,0 +1,34 @@
+#include <vector>
+#include <iostream>
+using std::vector;
+
+#include <algorithm>
+
+class Solution
+{
+public:
+    // 0 -> after buy for the first time
+    // 1 -> after sell for the first time
+    // 2 -> after buy for the second time
+    // 3 -> after sell for the second time
+
+    int maxProfit(vector<int> &prices)
+    {
+        int day_count = prices.size();
+
+        int state_0 = -prices[0];
+        int state_1 = 0;
+        int state_2 = -prices[0];
+        int state_3 = 0;
+
+        for (int day = 1; day < day_count; day++)
+        {
+            state_0 = std::max(state_0, -prices[day]);
+            state_1 = std::max(state_1, state_0 + prices[day]);
+            state_2 = std::max(state_2, state_1 - prices[day]);
+            state_3 = std::max(state_3, state_2 + prices[day]);
+        }
+
+        return std::max({state_0, state_1, state_2, state_3});
+    }
+};