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103. Binary Tree Zigzag Level Order Traversal

Difficulty Topics

Description

Given the root of a binary tree, return the zigzag level order traversal of its nodes' values. (i.e., from left to right, then right to left for the next level and alternate between).

 

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[20,9],[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].
  • -100 <= Node.val <= 100

Solution

binary-tree-zigzag-level-order-traversal.py
# 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 zigzagLevelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root: return []
        res = []
        r = 0
        queue = collections.deque([root])

        while queue:
            l = []
            n = len(queue)
            for _ in range(n):
                node = queue.popleft()
                l.append(node.val)

                for leaf in (node.left, node.right):
                    if leaf:
                        queue.append(leaf)

            if r: l.reverse()
            res.append(l)
            r ^= 1

        return res