added daily challenge

Author: Pasindu-Vishmika

543. Diameter of Binary Tree.py

@@ -0,0 +1,22 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def diameterOfBinaryTree(self, root: Optional[TreeNode]) -> int:
+        def diameter(node, res):
+            if not node:
+                return 0
+            left = diameter(node.left, res)
+            right = diameter(node.right, res)
+
+            res[0] = max(res[0], left + right)
+            
+            return max(left, right) + 1
+        
+        res = [0]
+        diameter(root, res)
+        return res[0]
+