Added C++ code for Diameter of Binary Tree (#257)

* Adding code of Diameter of Binary Tree for Cpp

* Update README.md

Author: iamarshsingh

README.md

@@ -216,7 +216,7 @@ LeetCode
 |572|[Subtree of Another Tree](https://leetcode.com/problems/subtree-of-another-tree/) | [Python](./algorithms/python/SubtreeOfAnotherTree/isSubtree.py)|Easy|
 |563|[Binary Tree Tilt](https://leetcode.com/problems/binary-tree-tilt/) | [Python](./algorithms/python/BinaryTreeTilt/findTilt.py)|Easy|
 |547|[Friend Circles](https://leetcode.com/problems/friend-circles/) | [C++](./algorithms/cpp/friendCircles/FriendCircles.cpp)|Medium|
-|543|[Diameter of Binary Tree](https://leetcode.com/problems/diameter-of-binary-tree/) | [Python](./algorithms/python/DiameterOfBinaryTree/diameterOfBinaryTree.py)|Easy|
+|543|[Diameter of Binary Tree](https://leetcode.com/problems/diameter-of-binary-tree/) | [C++](./algorithms/cpp/diameterOfBinaryTree/diameterOfBinaryTree.cpp), [Python](./algorithms/python/DiameterOfBinaryTree/diameterOfBinaryTree.py)|Easy|
 |538|[Convert BST to Greater Tree](https://leetcode.com/problems/convert-bst-to-greater-tree/) | [Python](./algorithms/python/ConvertBSTtoGreaterTree/convertBST.py)|Easy|
 |532|[K-diff Pairs in an Array](https://leetcode.com/problems/k-diff-pairs-in-an-array/) | [Python](./algorithms/python/K-diffPairsInAnArray/findPairs.py)|Easy|
 |520|[Detect Capital](https://leetcode.com/problems/detect-capital/) | [C++](./algorithms/cpp/detectCapital/DetectCapital.cpp)|Easy|

algorithms/cpp/diameterOfBinaryTree/diameterOfBinaryTree.cpp

@@ -0,0 +1,31 @@
+// Source : https://leetcode.com/problems/add-and-search-word-data-structure-design/
+// Author : Arshdeep Singh
+// Date   : 2021-10-01
+/**
+ * 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 Solution {
+private:
+    int diameter;
+    int maxDepth(TreeNode* root) {
+        if(root==NULL) return 0;
+        int left = maxDepth(root->left);
+        int right = maxDepth(root->right);
+        diameter = max(diameter, left + right);
+        return 1 + max(left, right);
+    }
+public:
+    int diameterOfBinaryTree(TreeNode* root) {
+        diameter = 0;
+        maxDepth(root);
+        return diameter;
+    }
+};