Binary Tree Right Side View

0199.Binary-Tree-Right-Side-View/memo.md

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+# 199. Binary Tree Right Side View
+
+## step1
+
+右の子を先に処理するin-orderで書ける。8mぐらい。
+
+nonlocal をつけずに一度間違えた。変数のスコープを意識できていないためなので反省。
+
+Mediumの問題の中ではスラスラと解けた方だと思う。
+
+## step2
+pre-order, bfsでも書ける。nonlocalな変数が不要になる。MAX_NODESも不要になるのでこちらが良さそう。
+
+その他、これ以降これ以降以下のように型を意識していきたい。
+- List[] -> list[]と書き直す
+- 空のコンテナには型をつける
+- 関数にも型をつける
+
+
+### 他の人のコード
+https://github.com/huyfififi/coding-challenges/pull/55
+
+上書きするようにすれば左を先に処理するpre-order, bfsでも書ける。
+
+
+
+

0199.Binary-Tree-Right-Side-View/step1.py

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+from typing import Optional
+
+
+# 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
+
+
+MAX_NODES = 100
+
+
+class Solution:
+    def rightSideView(self, root: Optional[TreeNode]) -> list[int]:
+        if root is None:
+            return []
+
+        result = [-1] * MAX_NODES
+        max_depth = 0
+
+        def traverse_right_first(node: Optional[TreeNode], depth: int) -> None:
+            nonlocal max_depth
+            if node is None:
+                return
+            traverse_right_first(node.right, depth + 1)
+            if result[depth] == -1:
+                result[depth] = node.val
+            max_depth = max(max_depth, depth)
+            traverse_right_first(node.left, depth + 1)
+
+        traverse_right_first(root, 0)
+        return result[: max_depth + 1]

0199.Binary-Tree-Right-Side-View/step2_bfs.py

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+from typing import Optional
+
+
+# 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 rightSideView(self, root: Optional[TreeNode]) -> list[int]:
+        if root is None:
+            return []
+
+        result: list[int] = []
+        frontier = [root]
+        depth = 0
+
+        while frontier:
+            next_frontier: list[Optional[TreeNode]] = []
+
+            for node in frontier:
+                if node is None:
+                    continue
+                if len(result) == depth:
+                    result.append(node.val)
+                next_frontier.append(node.right)
+                next_frontier.append(node.left)
+
+            frontier = next_frontier
+            depth += 1
+
+        return result

0199.Binary-Tree-Right-Side-View/step2_bfs_back.py

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+from typing import Optional
+
+
+# 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 rightSideView(self, root: Optional[TreeNode]) -> list[int]:
+        if root is None:
+            return []
+
+        result: list[int] = []
+        frontier = [root]
+
+        while frontier:
+            result.append(frontier[-1].val)
+
+            next_frontier: list[TreeNode] = []
+            for node in frontier:
+                if node.left is not None:
+                    next_frontier.append(node.left)
+                if node.right is not None:
+                    next_frontier.append(node.right)
+            frontier = next_frontier
+
+        return result

0199.Binary-Tree-Right-Side-View/step2_preorder.py

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+from typing import Optional
+
+
+# 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 rightSideView(self, root: Optional[TreeNode]) -> list[int]:
+        if root is None:
+            return []
+
+        result = []
+
+        def traverse(node: Optional[TreeNode], depth: int) -> None:
+            if node is None:
+                return
+
+            if len(result) == depth:
+                result.append(node.val)
+
+            traverse(node.right, depth + 1)
+            traverse(node.left, depth + 1)
+
+        traverse(root, 0)
+        return result

0199.Binary-Tree-Right-Side-View/step2_preorder_from_left_side_effects.py

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+from typing import Optional
+
+
+# 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 rightSideView(self, root: Optional[TreeNode]) -> list[int]:
+        if root is None:
+            return []
+
+        result: list[int] = []
+
+        def traverse_and_store_rightmost(node: Optional[TreeNode], depth: int) -> None:
+            if node is None:
+                return
+            if depth < len(result):
+                result[depth] = node.val
+            else:
+                result.append(node.val)
+            traverse_and_store_rightmost(node.left, depth + 1)
+            traverse_and_store_rightmost(node.right, depth + 1)
+
+        traverse_and_store_rightmost(root, 0)
+        return result