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What difficulty level is this coding question?

This is a Medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Meta.

What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Implement string dedup and mirror tree traversal

Quick Overview

This question evaluates string-processing and tree-traversal algorithm skills, focusing on efficient group-deletion and streaming-capable string dedup operations and a mirrored boundary traversal of binary trees, and it tests algorithmic design, complexity guarantees, and correctness/termination reasoning.

Company: Meta

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Technical Screen

- String cleanup with group deletions (LeetCode-inspired): Given a string s consisting of lowercase letters, repeatedly delete any maximal contiguous group of two or more identical characters (e.g., "abbbaaccz" -> remove "bbb" and "aa" -> "ccz" -> remove "cc" -> "z"). Return the final string after all deletions. Design an O(n) time, O(n) space solution and explain why it terminates and is correct. Follow-up: support a parameter k >= 2 to delete any group whose length is at least k; and support streaming input where characters arrive one by one. - Mirror boundary traversal of a binary tree (LeetCode-inspired): Given the root of a binary tree, output a "mirror boundary" list constructed as follows: ( 1) take the left boundary nodes (excluding the root) from the deepest leftmost leaf up to root.left (bottom-up); ( 2) then include root; ( 3) then take the right boundary nodes (excluding the root) from root.right down to the deepest rightmost leaf (top-down). Do not list any node more than once; if a node is both on a boundary and a leaf, include it once. Return the resulting list of node values. Provide an algorithm with time O(n) and space O(h), where h is the tree height.

Quick Answer: This question evaluates string-processing and tree-traversal algorithm skills, focusing on efficient group-deletion and streaming-capable string dedup operations and a mirrored boundary traversal of binary trees, and it tests algorithmic design, complexity guarantees, and correctness/termination reasoning.

String Cleanup By Group Deletions

Repeatedly delete groups whose length is at least k and return the final string.

Constraints

Inputs are Python literals matching the function signature.
Return a deterministic exact-match value.

Examples

Input: ('abbbaaccz', 2)

Expected Output: 'abz'

Explanation: Cascading deletions leave z.

Input: ('deeedbbcccbdaa', 3)

Expected Output: 'aa'

Explanation: k=3 deletion variant.

Input: ('abc', 2)

Expected Output: 'abc'

Explanation: No deletions.

Hints

Clarify edge cases before coding.
Keep the return value deterministic.

Mirror Boundary Traversal

Trees are [value,left,right]. Return left boundary bottom-up, root, and right boundary top-down without duplicates.

Constraints

Inputs are Python literals matching the function signature.
Return a deterministic exact-match value.

Examples

Input: ([1,[2,[4,None,None],[5,None,None]],[3,None,[6,None,None]]],)

Expected Output: [4, 2, 1, 3, 6]

Explanation: Left boundary bottom-up, root, then right boundary top-down.

Input: ([1,None,None],)

Expected Output: [1]

Explanation: Single root appears once.

Hints

Clarify edge cases before coding.
Keep the return value deterministic.

Quick Overview