problem: 0199 binary tree right side view

Author: cychiu8

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

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+# 0199. Binary Tree Right Side View
+
+Difficulty: medium
+Link: https://leetcode.com/problems/binary-tree-right-side-view/
+Topics: DFS-BFS
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT: TreeNode
+    - OUTPUT: List[int]
+
+# Naive Solution
+
+### Thought Process
+
+1. BFS
+2. 只記錄該層最右側的 node
+- Implement
+    
+    ```python
+    # 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 not root:
+                return []
+            result = []
+            q = collections.deque()
+            q.append(root)
+            
+            while q:
+                lenq = len(q)
+                for i in range(lenq):
+                    current = q.popleft()
+                    if current.left:
+                        q.append(current.left)
+                    if current.right:
+                        q.append(current.right)
+                    if i == lenq - 1:
+                        result.append(current.val)
+            return result
+    ```
+    
+
+### Complexity
+
+- Time complexity: O(N)
+- Space complexity: O(N)
+
+### Problems & Improvement
+
+- 是否可以直接看右邊的 node 即可
+    - 需思考這種類型的問題，如果一開始只放 right 的 node 即會出現未考慮到 4 的問題
+        
+        ![Untitled](./Untitled.png)
+        
+    - 使用 BFS 每一個 level 直接造訪是較 general 的方式