Binary tree zigzag level order traversal

Time: O(N); Space: O(N); medium

Given 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: {3,9,20,#,#,15,7}

  3
 / \
9  20
  /  \
 15   7

Output (zigzag level order traversal):

[
  [3],
  [20,9],
  [15,7]
]

[1]:

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

[2]:

class Solution1(object):
    def zigzagLevelOrder(self, root):
        '''
        :type root: TreeNode
        :rtype: List[List[int]]
        '''
        if root is None:
            return []
        result, current, level = [], [root], 1
        while current:
            next_level, vals = [], []
            for node in current:
                vals.append(node.val)
                if node.left:
                    next_level.append(node.left)
                if node.right:
                    next_level.append(node.right)
            if level % 2:
                result.append(vals)
            else:
                result.append(vals[::-1])
            level += 1
            current = next_level
        return result

[3]:

if __name__ == "__main__":

    root = TreeNode(3)
    root.left = TreeNode(9)
    root.right = TreeNode(20)
    root.right.left = TreeNode(15)
    root.right.right = TreeNode(7)

    assert Solution1().zigzagLevelOrder(root) == [[3], [20, 9], [15, 7]]