617. Merge Two Binary Trees

arai60/merge-two-binary-trees/README.md

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+## 考察
+- 初見の問題
+- 方針
+    - DFS/BFS
+        - 全ノードを探索しながら新しい二分木を生成して返すのが思い付く
+        - どちらかの二分木を再利用もできそうだが、新しい二分木を作る方を選択
+        - Pythonで関数からの戻り値をいじったら、他のところが壊れましたはびっくりしそう
+- 再帰の最大深さが2000だが、まずは再帰で書いてみる
+- Step2で非再帰に書き直そうと思う
+- あとは実装
+
+## Step1
+- 再帰DFSで実装
+- time: O(n), space: O(n)
+
+## Step2
+- 非再帰に書き換え
+    - 3点を書き換えた
+        - popの位置に再帰関数の処理を持ってくる
+        - 再帰呼び出しをstackへのappendに変更
+        - returnをcontinueへ変更
+    - stack -> queueとすればBFSにもなる
+- 他の人のPRを検索
+    - めちゃくちゃシンプル
+        - https://github.com/SuperHotDogCat/coding-interview/pull/35/files#diff-e23bfd258f7b29f3e94954b1beeca569cce5d7f3b17de6f3223729c1ebf588a5
+        - 帰りがけで下から構築していくのは思い付かなかった...
+        - これならhelperも作る必要ない
+    - こちらもめちゃくちゃシンプル
+        - https://github.com/fhiyo/leetcode/pull/25
+        - 行きがけで書くにしてももう少し条件を整理できそう
+- 帰りがけDFSで実装してみる
+
+## Step3
+- 1回目: 1m36s
+- 2回目: 1m15s
+- 3回目: 1m17s

arai60/merge-two-binary-trees/step1.py

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+# 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 mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root1 and not root2: return None
+        if not root1: return deepcopy(root2)
+        if not root2: return deepcopy(root1)
+
+        merged_root = TreeNode(root1.val + root2.val)
+        def merge_trees_helper(root1: Optional[TreeNode], root2: Optional[TreeNode], merged_root: TreeNode) -> None:
+            if not root1 and not root2: return
+            if not root1:
+                merged_root.left = deepcopy(root2.left)
+                merged_root.right = deepcopy(root2.right)
+                return
+            if not root2:
+                merged_root.left = deepcopy(root1.left)
+                merged_root.right = deepcopy(root1.right)
+                return
+
+            if root1.left or root2.left:
+                left_sum = 0
+                if root1.left: left_sum += root1.left.val
+                if root2.left: left_sum += root2.left.val
+                merged_root.left = TreeNode(left_sum)
+                merge_trees_helper(root1.left, root2.left, merged_root.left)
+
+            if root1.right or root2.right:
+                right_sum = 0
+                if root1.right: right_sum += root1.right.val
+                if root2.right: right_sum += root2.right.val
+                merged_root.right = TreeNode(right_sum)
+                merge_trees_helper(root1.right, root2.right, merged_root.right)
+
+        merge_trees_helper(root1, root2, merged_root)
+        return merged_root

arai60/merge-two-binary-trees/step2.py

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+# 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 mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root1 and not root2: return None
+        if not root1: return deepcopy(root2)
+        if not root2: return deepcopy(root1)
+
+        merged_root = TreeNode(root1.val + root2.val)
+        merged_root.left = self.mergeTrees(root1.left, root2.left)
+        merged_root.right = self.mergeTrees(root1.right, root2.right)
+        return merged_root

arai60/merge-two-binary-trees/step2_nonrecursive.py

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+# 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 mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root1 and not root2: return None
+        if not root1: return deepcopy(root2)
+        if not root2: return deepcopy(root1)
+
+        merged_tree = TreeNode(root1.val + root2.val)
+        nodes_to_visit = [(root1, root2, merged_tree)]
+        while nodes_to_visit:
+            root1, root2, merged_root = nodes_to_visit.pop()
+            if not root1 and not root2: continue
+            if not root1:
+                merged_root.left = deepcopy(root2.left)
+                merged_root.right = deepcopy(root2.right)
+                continue
+            if not root2:
+                merged_root.left = deepcopy(root1.left)
+                merged_root.right = deepcopy(root1.right)
+                continue
+
+            if root1.left or root2.left:
+                left_sum = 0
+                if root1.left: left_sum += root1.left.val
+                if root2.left: left_sum += root2.left.val
+                merged_root.left = TreeNode(left_sum)
+                nodes_to_visit.append((root1.left, root2.left, merged_root.left))
+
+            if root1.right or root2.right:
+                right_sum = 0
+                if root1.right: right_sum += root1.right.val
+                if root2.right: right_sum += root2.right.val
+                merged_root.right = TreeNode(right_sum)
+                nodes_to_visit.append((root1.right, root2.right, merged_root.right))
+
+        return merged_tree

arai60/merge-two-binary-trees/step3.py

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+# 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 mergeTrees(self, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root1 and not root2: return None
+        if not root1: return deepcopy(root2)
+        if not root2: return deepcopy(root1)
+
+        merged_root = TreeNode(root1.val + root2.val)
+        merged_root.left = self.mergeTrees(root1.left, root2.left)
+        merged_root.right = self.mergeTrees(root1.right, root2.right)
+        return merged_root