Leetcode: Binary Tree Zigzag Level Order Traversal

Question

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]
]
confused what “{1,#,2,3}” means? > read more on how binary tree is serialized on OJ.

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Analysis

This problem can be sees as an extension of the level order traversal. One flag variable is used to decide whether the current list will be reversed.

Solution

# 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]] """

        if root==None:
            return []

        queue, lst, cur, res = [root], [], [], []
        lastnum, curnum = 1, 0
        flag = 0      # if false, print all nodes in current level in the reverse order

        while queue!=[]:
            while lastnum>0:
                temp = queue[0]
                queue = queue[1:]
                lastnum -= 1
                lst.append(temp.val)

                if temp.left!=None:
                    queue.append(temp.left)
                    curnum += 1
                if temp.right!=None:
                    queue.append(temp.right)
                    curnum += 1

            if flag%2==1:
                newlst = []
                for elem in lst:
                    newlst = [elem] + newlst
            else:
                newlst = lst

            res.append(newlst)

            lastnum = curnum
            curnum = 0
            lst = []
            flag += 1

        return res

Error Path

1. forget to update lst to be [] after the inner while. keep an eye on it !
2. when reversing lst, write newlst = newlst +[elem] instead of newlst = [elem] + newlst.

Take home message

In python, if flag=1, and flag = ~flag. The flag will not be change to be 1 as expected, but changed to be -1.