LeetCode 筆記 - 103. Binary Tree Zigzag Level Order Traversal

# 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 []
        
        upper_layer = [root]
        lower_layer = []
        
        result = []
        
        level = 0
        while upper_layer:
            current_result = []
            for node in upper_layer:
                current_result.append(node.val)
                
                if node.left:
                    lower_layer.append(node.left)
                if node.right:
                    lower_layer.append(node.right)
            level += 1
            if level % 2 == 0:
                current_result = reversed(current_result)
            
            result.append(current_result)
            
            upper_layer = lower_layer
            lower_layer = []
        
        return result