How should I practice Binary Tree Spiral Level Order?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Bloomberg LP phone screen and onsite interview problems in FastPrep.

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Binary Tree Spiral Level Order

Bloomberg LP● MediumNEW GRADPHONE SCREENONSITE INTERVIEW

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Problem statement

Return the values of a binary tree one level at a time in alternating directions.

For this exercise, assume the root level is traversed left to right, the next level right to left, and directions alternate thereafter. Group values by their distance from the root and omit missing children. Preserve node positions even when values repeat. An empty tree returns an empty list.

Function

zigzagLevels(root: TreeNode) → int[][]

Examples

Example 1

root = [3,9,20,null,null,15,7]return = [[3],[20,9],[15,7]]

The root is left-to-right, the second level is right-to-left, and the third returns to left-to-right.

Example 2

root = [1,2,3,4,null,null,5]return = [[1],[3,2],[4,5]]

Null links are skipped; the second level is [3,2] and the third is [4,5].

Example 3

root = []return = []

An empty tree has no levels.

Constraints

For this exercise, assume a finite acyclic binary tree with 0 through 500 nodes.
-10^6 <= node.val <= 10^6.