Flatten nested JSON

Transform nested JSON objects into a flat dictionary using dot notation for keys.

json recursion dictionaries parsing

Write a function `flatten_json(data)` that takes a JSON-serializable object (dict, list, str, int, float, bool, None) and returns a flat dictionary where nested keys are joined with dots. For lists, use the index as the key segment (e.g., `"items.0"`). Empty objects and empty lists should be preserved as empty dict/list values. If the top-level value is not a dict or list, the key is an empty string `""`. If the top-level value is an empty dict, return an empty dictionary. If the top-level value is an empty list, return `{"": []}`. The result should be a plain `dict` with string keys.

Constraints

- Input is a valid JSON value (dict, list, primitive).
- Depth is at most 100.
- Keys are non-empty strings.
- List indices are integers starting at 0.
- Output must not contain any nested dict or list values, except as the value for empty objects/lists.
- Complexity: O(N) where N is total number of keys/elements.

Example

>>> flatten_json({"a": 1, "b": {"c": 2}})
{"a": 1, "b.c": 2}

>>> flatten_json({"a": [1, {"b": 2}]})
{"a.0": 1, "a.1.b": 2}

>>> flatten_json({})
{}

>>> flatten_json([])
{"": []}

20 points ~25 min

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Hints

Use a helper recursive function that tracks the current key prefix.

When encountering a list, iterate with index and append the index as a key segment.

Empty dict/list should not be recursed into; store them as values directly.

The top-level key prefix starts as an empty string; handle the case where the root is an empty dict separately.

Test Results

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