# Amazon's Most-Feared Tree Traversal (and its simple solution)

## Метаданные

- **Канал:** StrataScratch
- **YouTube:** https://www.youtube.com/watch?v=ylpi5jNCSVA
- **Дата:** 16.04.2026
- **Длительность:** 3:39
- **Просмотры:** 168
- **Источник:** https://ekstraktznaniy.ru/video/51758

## Описание

Amazon asked this Binary Tree Zigzag Level Order Traversal question and 90% of candidates panic. Here's the clean, elegant solution that actually works under interview pressure - BFS + conditional reversal. No two-stack madness. No over-engineering.

In this video, I break down:
✅ What zigzag traversal actually is (with a clear example)
✅ Why most candidates overcomplicate it
✅ The one insight that makes this problem easy
✅ Step-by-step code walkthrough (Python)
✅ 4 interview tips to separate yourself from 80% of other candidates

🔑 KEY INSIGHT: Zigzag is just normal BFS traversal with post-processing - reverse the odd levels after collecting them. That's it.

This approach is simple to code, easy to explain, and bulletproof under pressure.

If you're prepping for Amazon, Google, or any FAANG interview, this one's essential.
👍 Like if this helped 🔔 Subscribe for more interview problems broken down clearly

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## Транскрипт

### The zigzag trap []

This interview question looks like a basic breadth-first search or BFS algorithm. Then Amazon adds zigzag and everything falls apart. Here's what to do when zigzag shows up in your interview.

### What is zigzag traversal? [0:11]

interview. This tip alone is worth the entire video. Zigzag traversal alternates direction on each level. Left to right, then right to left, then left to right. Consider a tree with root three, left child nine, right child 20, and node 20 has left child 15 and right child seven. In normal level order, we'd get three, then nine and 20, then 15 and seven. But in zigzag order, we get three, then 20 and nine. Notice we reversed the second level, then 15 and seven. Here's where 90% of candidates crash and

### Why 90% of candidates crash [0:42]

burn. They overthink it, trying two stacks, complex reversal logic during traversal. No need for that. There's an elegant solution, regular BFS with conditional reversal. Use a queue

### The elegant solution: BFS + conditional reversal [0:53]

for BFS, track the level, reverse odd levels. That's it. The key insight, zigzag is just normal traversal with post-processing. Let me walk you through the algorithm step-by-step. The first three code parts are tree node class defines our tree structure. List to tree converts the array input into an actual tree, and zigzag level order is where the magic happens. I'll focus on that third part. First, we handle the edge case. If the tree is empty, we return an empty list. Then we initialize our result list, a queue containing just the root node, and a level counter starting at zero. Now comes the main loop. While our queue isn't empty, we're going to process exactly one level at a time. This is crucial. We capture the current size of the queue, which tells us how many nodes are in the current level. We create an empty list to store this level's values. For each node in the current level, we remove it from the front of the queue, add its value to our level values list, and then add its children, first left, then right, to the back of the queue. This is the standard BFS procedure that processes nodes in left-to-right order within each level. After we've collected all the values for the current level, we check if the level number is odd. If it is, we reverse the level values list. This gives us the zigzag effect. Even levels stay left to right, odd levels become right to left. Finally, we append the level values to our result and increment the level counter. The fourth and final part ties everything together. The wrapper converts input data to a tree, runs the algorithm, and returns the result. All that gives you this final code.

### The zigzag trick: reversing odd levels [2:32]

The beauty of this approach is its simplicity. We're leveraging the well-understood BFS pattern and adding just one conditional reversal step. Before you ask, yes, there are other ways to solve this. Two stacks

### Why to ignore two-stack solutions [2:44]

decks, whatever. But they're harder to code correctly under pressure, so you better ignore them. Now, for some interview tips. One, start with an

### 4 interview tips to stand out [2:52]

example. Draw the tree, show the zigzag pattern. This single step separates you from 80% of other candidates who jump straight to code. Two, always mention BFS first. Shows you understand the fundamentals before diving into the twist. Three, watch your level tracking. Odd levels get reversed. Off-by-one errors here will destroy you. Four, test edge cases. Single node, empty tree, left-heavy trees. Miss this testing step and even perfect code will fail you. That's zigzag traversal conquered. BFS plus conditional

### Recap & wrap-up [3:27]

reversal, elegant, simple, bulletproof. If you want more interview problems broken down this clearly, subscribe and I'll keep turning panic into confidence.