103 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,#,#,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]]
        """
        result = []
        if not root:
            return result
        curr_level = [root]
        need_reverse = False
        while curr_level:
            level_result = []
            next_level = []
            for temp in curr_level:
                level_result.append(temp.val)
                if temp.left:
                    next_level.append(temp.left)
                if temp.right:
                    next_level.append(temp.right)
            if need_reverse:
                level_result.reverse()
                need_reverse = False
            else:
                need_reverse = True
            result.append(level_result)
            curr_level = next_level
        return result


if __name__ == "__main__":
    None