Deep Object Filter - Solution & Explanation

Problem Overview: Deep Object Filter asks you to recursively process a nested structure (objects and arrays) and remove values that do not satisfy a predicate function fn. Primitive values are checked directly with the predicate, while arrays and objects must be traversed deeply so that only valid elements remain in the final structure.

Approach 1: Recursive DFS Traversal (O(n) time, O(h) space)

The most natural solution treats the input as a tree of values and performs a depth‑first traversal. If the current value is an array, iterate through each element, recursively filter it, and keep only the results that are still valid. If the value is an object, iterate through its keys and recursively filter each property, deleting keys whose filtered result is invalid. For primitives, simply evaluate fn(value) and return the value only when the predicate passes. This approach processes each node exactly once, giving O(n) time complexity where n is the number of values in the structure, and O(h) recursion stack space where h is the maximum nesting depth. This pattern is common in problems involving recursion and nested data traversal.

Approach 2: Iterative Stack Traversal (O(n) time, O(n) space)

You can replace recursion with an explicit stack to simulate depth‑first traversal. Push the root node onto a stack along with a reference to its parent container. While the stack is not empty, pop a node, inspect its type, and process arrays or objects by pushing their children back onto the stack. After evaluating children, remove entries that fail the predicate or become empty after filtering. This avoids recursion depth limits and can be easier to control in environments where stack overflow is a concern. The algorithm still visits every element once, resulting in O(n) time complexity and up to O(n) auxiliary space for the stack and intermediate references. The traversal pattern is similar to iterative depth-first search used for tree structures.

Recommended for interviews: The recursive DFS approach is what most interviewers expect. It shows you understand how to traverse nested data structures and apply transformations during recursion. Mentioning the iterative stack variant demonstrates deeper understanding of traversal mechanics and recursion limits. If the discussion touches JavaScript behavior, highlighting how arrays and objects are processed differently in JavaScript can also strengthen your explanation.