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

# Time:  O(n)
# Space: O(n)
#
# 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,#,#,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):
    # @param root, a tree node
    # @return a list of lists of integers
    def zigzagLevelOrder(self, root):
        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