Leetcode #2631: Group By
In this guide, we solve Leetcode #2631 Group By in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Write code that enhances all arrays such that you can call the array.groupBy(fn) method on any array and it will return a grouped version of the array. A grouped array is an object where each key is the output of fn(arr[i]) and each value is an array containing all items in the original array which generate that key.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: JavaScript
Intuition
The constraints allow a direct scan that keeps only the essential state.
By translating the requirements into a clean loop, the logic stays easy to reason about.
Approach
Iterate through the data once, updating the state needed to compute the answer.
Return the final state after the traversal is complete.
Steps:
- Parse the input.
- Iterate and update state.
- Return the computed answer.
Example
Input:
array = [
{"id":"1"},
{"id":"1"},
{"id":"2"}
],
fn = function (item) {
return item.id;
}
Output:
{
"1": [{"id": "1"}, {"id": "1"}],
"2": [{"id": "2"}]
}
Explanation:
Output is from array.groupBy(fn).
The selector function gets the "id" out of each item in the array.
There are two objects with an "id" of 1. Both of those objects are put in the first array.
There is one object with an "id" of 2. That object is put in the second array.
Python Solution
# TODO: add Python solution
Complexity
The time complexity is O(n). The space complexity is O(1) to O(n).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.