[LeetCode] 103. Binary Tree Zigzag Level Order Traversal

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:

# 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]]:
        
        levels = []
        
        if root is None:
            return []
        
        def levelOrder(root, level):
            if level == len(levels):
                levels.append([])
            
            if level % 2 == 0:
                levels[level].append(root.val)
            else:
                levels[level].insert(0,root.val)
            
            
            if root.left:
                levelOrder(root.left, level+1)
            if root.right:
                levelOrder(root.right, level+1)
                
        
        levelOrder(root,0)
        
        return levels