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Decode the Coding Interview in C++: Real-World Examples

Binary Tree Zigzag Level Order Traversal

Explore the method for traversing a binary tree in a zigzag pattern, alternating from left to right and right to left at each level. Learn to apply breadth-first search with a deque to efficiently collect nodes in the desired order while managing time and space complexity.

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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

Files
main.cpp
TreeNode.h
C++ 17
#include "TreeNode.h"
std::vector<std::vector<int>> zigzagLevelOrder(TreeNode* root) {
    return {};        
}
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 ...