Flatten nested JSON into a string map

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).