Leetcode #2775: Undefined to Null
In this guide, we solve Leetcode #2775 Undefined to Null in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given a deeply nested object or array obj, return the object obj with any undefined values replaced by null. undefined values are handled differently than null values when objects are converted to a JSON string using JSON.stringify().
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: JavaScript
Intuition
We need to traverse every value in a potentially nested structure and replace only the undefined values.
A depth-first walk mirrors the structure naturally, letting us rebuild lists and dictionaries while keeping other values intact.
Approach
Recursively walk the input.
When we encounter the JavaScript-style undefined, replace it with None (Python's null). For lists and dictionaries, rebuild them by applying the same conversion to each child.
Steps:
- If the value is
undefined, returnNone. - If it's a list, map the conversion over all elements.
- If it's a dict, convert every value.
- Otherwise, return the value unchanged.
Example
Input: obj = {"a": undefined, "b": 3}
Output: {"a": null, "b": 3}
Explanation: The value for obj.a has been changed from undefined to null
Python Solution
class Undefined:
pass
UNDEFINED = Undefined()
class Solution:
def undefinedToNull(self, obj):
if obj is UNDEFINED:
return None
if isinstance(obj, list):
return [self.undefinedToNull(x) for x in obj]
if isinstance(obj, dict):
return {k: self.undefinedToNull(v) for k, v in obj.items()}
return obj
Complexity
The time complexity is , and the space complexity is due to recursion and rebuilt structures.
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.