문제 설명

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

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

Example 2:

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

Example 3:

Input: root = []
Output: []

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -1000 <= Node.val <= 1000

문제풀이

class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        
        traversal_queue, result = [root], []
    
        while traversal_queue:
            result.append([node.val for node in traversal_queue])
            child_pair = [(node.left, node.right) for node in traversal_queue]
            traversal_queue = [ child for pair in child_pair for child in pair if child ]
            
        return result

참고 : https://leetcode.com/problems/binary-tree-level-order-traversal/