Undefined to Null — LeetCode 2775 Python Solution

The problem

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().

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

Pattern: Depth-First Search

Follow one path to its end before trying the next — the default way to explore a graph. LeetCode 2775. Undefined to Null is filed here because the reference solution below belongs to the algorithm family this hub collects, even though its LeetCode tags point elsewhere.

The depth-first search guide has the Python template for the pattern and the 366 LeetCode problems that use it.

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