add solution: maximum-depth-of-binary-tree

Author: dusunax

maximum-depth-of-binary-tree/dusunax.py

@@ -0,0 +1,38 @@
+'''
+# 104. Maximum Depth of Binary Tree
+
+use DFS to find the maximum depth of the binary tree.
+
+``` 
+helper function explanation:
+- store the max_depth in the nonlocal variable.(outside of the helper function)
+- base case: if the node is None, update the max_depth and return.
+- in the helper function, do recursive call for the left and right children of the node.
+  - update the count for the depth of the tree.
+- update the max_depth when the node is a leaf node's children.
+```
+
+## TC is O(n)
+
+visit each node once for checking if it is a leaf node's children.
+
+## SC is O(h)
+
+h for height of the tree
+'''
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int: 
+        max_depth = 0
+
+        def helper(node, count):
+            nonlocal max_depth 
+            if node is None:
+                max_depth = max(max_depth, count)
+                return
+
+            helper(node.left, count+1) 
+            helper(node.right, count + 1)
+
+        helper(root, max_depth)
+
+        return max_depth