Grokking the Coding Interview: Patterns for Coding Questions
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Solution: Zigzag Traversal
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Problem Statement

Given a binary tree, populate an array to represent its zigzag level order traversal. You should populate the values of all nodes of the first level from left to right, then right to left for the next level and keep alternating in the same manner for the following levels.

Example 1:

Image

Example 2:

Image

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -1000 <= Node.val <= 1000

Solution

This problem follows the Binary Tree Level Order Traversal pattern. We can follow the same BFS approach. The only additional step we have to keep in mind is to alternate the level order traversal, which means that for every other level, we will traverse similar to Reverse Level Order Traversal.

Here is the visual representation of the algorithm:

Zigzag Traversal
Zigzag Traversal

Code

Here is the code for this algorithm:

Python3
Python3
. . . .

Time Complexity

The time complexity of the above algorithm is O(N), where ‘N’ is the total number of nodes in the tree. This is due to the fact that we traverse each node once.

Space Complexity

The space complexity of the above algorithm will be O(N) as we need to return a list containing the level order traversal. We will also need O(N) space for the queue. Since we can have a maximum of N/2 nodes at any level (this could happen only at the lowest level), therefore we will need O(N) space to store them in the queue.

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