Add C++ solution of problem #1261

Author: Cuda-Chen

1261/solution.cpp

@@ -0,0 +1,40 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class FindElements {
+public:
+    FindElements(TreeNode* root) {
+        f(root, 0);
+    }
+    
+    bool find(int target) {
+        return m.count(target);
+    }
+
+private:
+    TreeNode *root;
+
+    void f(TreeNode *t, int idx) {
+        if(!t)
+            return;
+        m.insert(idx);
+        f(t->left, idx * 2 + 1);
+        f(t->right, idx * 2 + 2);
+    }
+
+    unordered_set<int> m;
+};
+
+/**
+ * Your FindElements object will be instantiated and called as such:
+ * FindElements* obj = new FindElements(root);
+ * bool param_1 = obj->find(target);
+ */