LeetCode/103_Binary_Tree_Zigzag_Level_Order_Traversal.py at main · AnmolPatel20/LeetCode · GitHub

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"""
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

"""

# 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]]:
        res = []
        q = deque([root] if root else[])
        while q:
            level = []
            for i in range(len(q)):
                node = q.popleft()
                level.append(node.val)
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)
            level = list(reversed(level)) if len(res) % 2 else level
            res.append(level)
        return res