Binary Tree Zigzag Level Order Traversal.py

'''
Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

For example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its zigzag level order traversal as:
[
  [3],
  [20,9],
  [15,7]
]
'''



# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        if not root:
            return res
        zigzag = True
        vec = [root]
        while len(vec):
            zigzag = not zigzag
            next_vec = []
            tmp = []
            for node in vec:
                if zigzag:
                    tmp = [node.val] + tmp
                else:
                    tmp.append(node.val)
                    
                if node.left:
                    next_vec.append(node.left)
                if node.right:
                    next_vec.append(node.right)
            res.append(tmp)
            
            vec = next_vec
        return res