O(n) time and O(n) space for implicit stack using recursion

Author: 149ps

104. Maximum Depth of Binary Tree/104. Maximum Depth of Binary Tree.py

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+"""
+Given a binary tree, find its maximum depth.
+
+The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
+
+Note: A leaf is a node with no children.
+
+Example:
+
+Given binary tree [3,9,20,null,null,15,7],
+
+    3
+   / \
+  9  20
+    /  \
+   15   7
+return its depth = 3.
+"""
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution:
+    def maxDepth(self, root: TreeNode) -> int:
+        d = 0
+        def depth(node):
+            nonlocal d
+            if not node:
+                return 0
+            left = depth(node.left)
+            right = depth(node.right)
+            d = max(d,left+right)
+            return max(left,right)+1
+        return depth(root)