Practice/Google/Leetcode 2675. Array of Objects to Matrix

Leetcode 2675. Array of Objects to Matrix

CodingOptional

Problem

You are building a data export feature that converts a collection of JavaScript objects into a spreadsheet-like matrix format. Given an array of objects, transform it into a 2D array where:

• The first row contains all unique keys from across all objects, ordered by their first appearance
• Each subsequent row represents one object from the input array
• For each row, values are placed in columns corresponding to their keys
• If an object is missing a key that exists in the header, use an empty string ("") as a placeholder

This is useful for exporting JSON data to CSV format or displaying object collections in tabular form.

Requirements

1. Extract all unique keys across all objects in order of first appearance
2. Create a header row containing these keys
3. For each object, create a data row with values aligned to the header columns
4. Use empty strings ("") for missing key-value pairs
5. Preserve the original data types of values (numbers, strings, nested objects, etc.)

Constraints

• 0 ≤ array length ≤ 1000
• Each object can have 0 to 100 keys
• Keys are strings
• Values can be of any JSON-serializable type (numbers, strings, booleans, objects, arrays, null)
• Key ordering must follow first appearance across all objects sequentially
• Time complexity should be O(n × k) where n is the number of objects and k is the average number of keys
• Space complexity should be O(n × m) where m is the total number of unique keys

Examples

Example 1:

Input: [{"name": "Alice", "age": 30}, {"name": "Bob", "age": 25}] Output: [["name", "age"], ["Alice", 30], ["Bob", 25]] Explanation: Both objects have identical keys. The header row is ["name", "age"], and each object becomes a data row.

Example 2:

Input: [{"a": 1, "b": 2}, {"b": 3, "c": 4}] Output: [["a", "b", "c"], [1, 2, ""], ["", 3, 4]] Explanation: Keys appear in order: "a" (from obj1), "b" (from obj1), "c" (from obj2). First object lacks "c", second lacks "a".

Example 3:

Input: [] Output: [] Explanation: An empty input array results in an empty matrix.

Example 4:

Input: [{"x": 1}, {"y": 2}, {"z": 3}] Output: [["x", "y", "z"], [1, "", ""], ["", 2, ""], ["", "", 3]] Explanation: Each object has a unique key. Most cells will be empty strings.