binary-tree-zigzag-level-order-traversal.py

# 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).
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# Example 1:
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# Input: root = [3,9,20,null,null,15,7]
# Output: [[3],[20,9],[15,7]]
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# Example 2:
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# Input: root = [1]
# Output: [[1]]
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# Example 3:
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# Input: root = []
# Output: []
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# Constraints:
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# 	The number of nodes in the tree is in the range [0, 2000].
# 	-100 <= Node.val <= 100
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# 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: TreeNode) -> List[List[int]]:
        if root == None:
            return []
        self.zigzag = True
        queue = [root]
        res = []
        while queue:
            nextLevel = []
            level = [node.val for node in queue]
            if self.zigzag:
                res.append(level)
            else:
                res.append(level[::-1])
            for node in queue:
                if node.left: 
                    nextLevel.append(node.left)
                if node.right: 
                    nextLevel.append(node.right)
            if self.zigzag == True:
                self.zigzag = False
            else:
                self.zigzag = True
            queue = nextLevel
        return res