Flatten nested JSON into a string map

Q: Flatten nested JSON into a string map

This question evaluates proficiency with JSON data structures, hierarchical traversal, and serialization, testing competency in handling nested objects and arrays, primitive values, and edge cases such as deep nesting and empty containers.

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Question

Problem

You are given an input JSON object that may contain nested objects and arrays. Your task is to flatten it into a single-level key/value mapping, then serialize that mapping into an output string.

Flattening rules

Each leaf value (string/number/boolean/null) becomes one entry in the flattened map.
Keys in the flattened map represent the full path from the root:
- Use . to separate nested object keys.
- Use [i] to represent array indices.
The order of entries in the output does not matter.

Input

A JSON object (as a string or parsed structure).

Output

A single string representing the flattened mapping (e.g., JSON string of a flat object, or key=value pairs joined by newlines; pick one and document it).

Example

Input JSON:

{
  "a": 1,
  "b": {
    "c": 2,
    "d": [3, 4]
  },
  "e": [{"f": 5}]
}

One valid flattened result (as a flat JSON object) would be:

{
  "a": 1,
  "b.c": 2,
  "b.d[0]": 3,
  "b.d[1]": 4,
  "e[0].f": 5
}

Constraints / Notes

Depth may be large; avoid stack overflow if possible.
Values can be primitive JSON types; nested containers are objects/arrays.
Handle empty objects/arrays consistently (define whether they produce entries or are skipped).