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Decode the Coding Interview in Python: Real-World Examples

Binary Tree Zigzag Level Order Traversal

Explore the binary tree zigzag level order traversal technique, which alternates the traversal direction at each level. Understand how to implement this using a breadth-first search approach combined with a deque for efficient node insertion and removal. This lesson helps you master a common tree traversal method frequently asked in coding interviews.

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Description

Given a binary tree T, you have to find its nodes’ zigzag level order traversal. The zigzag level order traversal corresponds to traversing nodes from left to right and then right to left for the next level, alternating between.

You have to return the elements in each level in a two-dimensional array.

Let’s look at an example:

Coding exercise

Python 3.5
from TreeNode import *
def zigzagLevelOrder(root):
    pass
Binary Tree Zigzag Level Order Traversal

Solution

As per the problem statement, we need to traverse the tree in a level-by-level zigzag order. An intuitive approach for this problem would be to use Breadth-First Search (BFS). By default, BFS provides ordering from left to right within a single level.

We will need to modify BFS a little to get our desired output, a zigzagzigzag ...