102. Binary Tree Level Order Traversal

Given the root of a binary tree, return the level order traversal of its nodes’ values. (i.e., from left to right, level by level).

Example 1:

1 2	Input: root = [3,9,20,null,null,15,7] Output: [[3],[9,20],[15,7]]

Example 2:

1 2	Input: root = [1] Output: [[1]]

Example 3:

1 2	Input: root = [] Output: []

依層遍歷樹，用List Comprehension。

# 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: TreeNode) -> List[List[int]]:
        
        if not root: return []        
        ans = []
        level = [root]
        
        while level:            
            ans.append([node.val for node in level])
            level = [kid for node in level for kid in (node.left, node.right) if kid]        
        return ans

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