Q2-10 Maximum Depth of Binary Tree

md/2-10_MaximumDepthofBinaryTree.md

@@ -0,0 +1,87 @@
+# 2-10_MaximumDepthofBinaryTree
+Author: WaveAlchemist
+Given the root of a binary tree, return its maximum depth.
+
+A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
+
+# 1st-2nd
+Diameter of Binary Treeで実装した再帰を思い出して解答したが
+return 0のところを1としてしまい不正解になってしまった
+後でDiameter of Binary Treeを見て修正した
+
+``` Python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        left_depth = self.maxDepth(root.left)
+        right_depth = self.maxDepth(root.right)
+        depth = max(left_depth, right_depth) + 1
+        return depth
+```
+# 3rd
+discord上のほかの人が解答されたコードを見て自分で再構築
+- https://github.com/fhiyo/leetcode/pull/23/files
+    - 幅優先探索　 BFS
+    - 深さ優先探索 DFS
+- https://discordapp.com/channels/1084280443945353267/1227073733844406343/1236695050902048899
+    - 
+
+``` Python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        def append_node(nodes, node):
+            if not node:
+                return
+            nodes.append(node)
+        if not root:
+            return 0
+        depth = 0
+        nodes = [root]
+        while nodes:
+            next_depth_nodes = []
+            for node in nodes:
+                append_node(next_depth_nodes, node.left)
+                append_node(next_depth_nodes, node.right)
+            nodes = next_depth_nodes
+            depth += 1
+        return depth
+```
+
+``` Python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        if not root:
+            return 0
+        max_depth = 0
+        nodes_with_depth = [(root, 1)]
+        while nodes_with_depth:
+            node, depth = nodes_with_depth.pop()
+            max_depth = max(depth, max_depth)
+            if node.left:
+                nodes_with_depth.append((node.left, depth + 1))
+            if node.right:
+                nodes_with_depth.append((node.right, depth + 1))
+        return max_depth
+```
+
+# 4th
+odaさんのコメントをもとに変更
+pop してから、not node を確認する手もあるでしょう。その場合は、先頭の if not root もいらないかもしれません。
+
+``` Python
+class Solution:
+    def maxDepth(self, root: Optional[TreeNode]) -> int:
+        max_depth = 0
+        nodes_with_depth = [(root, 1)]
+        while nodes_with_depth:
+            node, depth = nodes_with_depth.pop()
+            if not node:
+                return 0
+            max_depth = max(depth, max_depth)
+            if node.left:
+                nodes_with_depth.append((node.left, depth + 1))
+            if node.right:
+                nodes_with_depth.append((node.right, depth + 1))
+        return max_depth
+```