Breadth-First Search/Binary Tree Zigzag Level Order TraversalMedium

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Binary Tree Zigzag Level Order Traversal

Breadth-First Search·Medium

Given the root of a binary tree, return the zigzag level order traversal of its node values. (i.e., from left to right, then right to left for the next level and alternate between).

Input convention. Level-order array with null for missing children. The starter includes TreeNode and arrayToTree. The natural implementation is BFS with a per-level reversal flag.

Examples

Input: values = [3,9,20,null,null,15,7]

Output: [[3],[20,9],[15,7]]

Input: values = [1]

Output: [[1]]

Input: values = []

Output: []

Constraints

The number of nodes is in the range [0, 2000].
-100 <= Node.val <= 100

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Tests

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Passed32 ms

First attempt — retry to track metric movement here.

Your approach

Pattern:Breadth-First Search
Time:O(N)
Space:O(1)

Complete approach — pattern and complexity both named.

Coach

What you got right

You correctly matched the sorted input requirement and implemented a linear-time scan using two pointers.

Where it diverged

No divergence detected. The code correctly aligns with the stated approach.

Next attempt: focus on

Try solving related sliding window problems to build familiarity with two-pointer variants.

Signals

No signals fired — clean run, you stayed in flow.