Leetcode (Python): Binary Tree Zigzag Level Order Traversal

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]
]

Solution

We use a variant of BFS with two stacks, one for this level one for the next one. As we traverse one level we add the children into the next level stack, once a level is traversed the next level stack becomes the actual one.

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

class Solution:
    # @param root, a tree node
    # @return a list of lists of integers
    def zigzagLevelOrder(self, root):
        solution = []
        thisLevel =[]
        if root != None:
            thisLevel.append(root)
        leftToRight = True
        while len(thisLevel)>0:
            levelSolution = []
            nextLevel = []
            while len(thisLevel)>0:
                node = thisLevel.pop()
                levelSolution.append(node.val)
                if leftToRight:
                    if node.left != None:
                        nextLevel.append(node.left)
                    if node.right != None:
                        nextLevel.append(node.right)
                else:
                    if node.right != None:
                        nextLevel.append(node.right)
                    if node.left != None:
                        nextLevel.append(node.left)
            thisLevel = nextLevel
            solution.append(levelSolution)
            leftToRight = not leftToRight
        return solution