problem: 0102 binary tree level order traversal

0102 Binary Tree Level Order Traversal/0102 Binary Tree Level Order Traversal.md

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+# 0102. Binary Tree Level Order Traversal
+
+* Difficulty: medium
+* Link: https://leetcode.com/problems/binary-tree-level-order-traversal/
+* Topics: DFS-BFS
+
+# Clarification
+
+1. Check the inputs and outputs
+    - INPUT: Optional[TreeNode]
+    - OUTPUT: List[List[int]
+
+# Naive Solution
+
+### Thought Process
+
+1. BFS
+2. get the nodes from level to level
+- 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 levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+
+            if not root:
+                return []
+
+            result = []
+            q = collections.deque()
+            q.append(root)
+
+            while q:
+                lenq = len(q)
+                r = []
+                for i in range(lenq):
+                    current = q.popleft()
+                    r.append(current.val)
+                    if current.left:
+                        q.append(current.left)
+                    if current.right:
+                        q.append(current.right)
+                result.append(r)
+
+            return result
+    ```
+
+
+### Complexity
+
+- Time complexity: $O(N)$
+    - 遍歷每個節點一次
+- Space complexity: $O(N)$
+    - queue 最大為所有 node 個數