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Palo Alto Networks·Software Engineer·Onsite - Coding / Algorithms·Intermediate

Intermediate
Jul 2026

Summary

Palo Alto Networks software engineer interview, coding round focused entirely on tree manipulation. Pretty standard algorithmic stuff but they pushed hard on complexity analysis and edge cases, which I wasn't fully prepared for.

Questions Asked (1)

Q1

Given a binary tree's root, mirror the tree by swapping every node's left and right children. Implement both a recursive and an iterative version, analyze time and space complexity for each, handle edge cases like an empty tree or a single node, and walk through a dry run on [5,3,8,null,4,7,9].

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

I got the recursive version out pretty fast, base case plus swap plus recurse, no big deal.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then present both recursive and iterative solutions with clear code or pseudocode. Analyze time and space complexity for each, and finally walk through a dry run on the given example to demonstrate correctness.

Pro tip: Mention that the iterative solution can use a stack (DFS) or queue (BFS) and discuss the trade-offs; this shows you understand multiple approaches and can choose based on constraints.

1. Clarify and outline

Restate the problem to ensure understanding, and explicitly list edge cases like empty tree and single node. Outline that you will provide both recursive and iterative solutions.

2. Recursive solution

Explain the recursive approach: swap left and right children, then recursively mirror the left and right subtrees. Provide concise code or pseudocode.

3. Iterative solution

Describe an iterative approach using a stack (or queue) to traverse the tree, swapping children at each node. Provide code or pseudocode.

4. Complexity analysis

Analyze time and space complexity for both solutions. Time is O(n) for both; space is O(h) for recursion (call stack) and O(n) for iterative (stack/queue), where h is tree height.

5. Dry run on example

Walk through the given tree [5,3,8,null,4,7,9] step by step, showing how nodes are swapped to produce the mirrored tree.

Key Points to Mention

  • Time complexity is O(n) for both recursive and iterative solutions because each node is visited once.
  • Space complexity: recursive uses O(h) call stack, iterative uses O(n) for stack/queue in worst case (skewed tree).
  • Edge cases: empty tree returns null; single node remains unchanged.
  • The iterative solution can use DFS (stack) or BFS (queue); both are valid but may have different space characteristics.
  • In-place modification is possible; no need to create new nodes.
  • Dry run should show the tree structure before and after mirroring, including null children.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.