Add C++ solution of problem #3392

Author: Cuda-Chen

3392/solution.cpp

@@ -0,0 +1,14 @@
+class Solution {
+public:
+    int countSubarrays(vector<int>& A) {
+        int ans = 0;
+        for(int i = 0; i < A.size() - 2; i++) {
+            int a = A[i];
+            int b = A[i + 1];
+            int c = A[i + 2];
+            if(2 * (a + c) == b)
+                ans++;
+        }
+        return ans;
+    }
+};