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How to Solve Group By Problem
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Start Free for WindowsMedium#2631
LeetCode ProblemGroup By
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. The provided callback fn will accept an item in the array and return a string key. The order of each value list should be the order the items appear in the array. Any order of keys is acceptable. Please solve it without lodash's _.groupBy function.
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Problem Breakdown
Understanding the Group By Problem
Let's break down this LeetCode problem and understand what makes it challenging in interview settings.
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. The provided callback fn will accept an item in the array and return a string key. The order of each value list should be the order the items appear in the array. Any order of keys is acceptable. Please solve it without lodash's _.groupBy function.
MediumProblem #2631
LeetCodeGroup By
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How Phantom Code Helps
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Examples
# Example 1
Input
array = [ {"id":"1"}, {"id":"1"}, {"id":"2"} ], fn = function (item) { return item.id; }Output
{ "1": [{"id": "1"}, {"id": "1"}], "2": [{"id": "2"}] }# Example 2
Input
array = [ [1, 2, 3], [1, 3, 5], [1, 5, 9] ] fn = function (list) { return String(list[0]); }Output
{ "1": [[1, 2, 3], [1, 3, 5], [1, 5, 9]] }# Example 3
Input
array = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] fn = function (n) { return String(n > 5); }Output
{ "true": [6, 7, 8, 9, 10], "false": [1, 2, 3, 4, 5] }Constraints
0 <= array.length <= 105
fn returns a string
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Solve Group By — Write code that enhances all arrays such that you can call the array.groupBy(fn)...
Here's the optimal approach using the right algorithm:
def solve(input):
# Optimal O(n) solution
return result
Time: O(n) | Space: O(n)
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Frequently Asked Questions
Common questions about solving Group By and using Phantom Code during coding interviews.
How does Phantom Code help me solve coding problems like Group By during interviews?
Phantom Code generates complete solutions instantly with proper complexity analysis, letting you focus on explaining your approach and demonstrating problem-solving skills rather than getting stuck on implementation details during high-pressure situations.
What if the interviewer asks follow-up questions or modifications to problems like Group By?
Phantom Code adapts in real-time. Screenshot the modified problem or use audio mode to capture the interviewer's follow-up, and get an updated solution within seconds — including edge cases and optimizations.
Can Phantom Code help me communicate my solution for problems like Group By?
Yes. Phantom Code provides step-by-step approach explanations with 3 progressive thoughts, so you can walk through your solution naturally. It also provides time and space complexity analysis to discuss trade-offs.
How does Phantom Code assist with different programming languages for Group By type problems?
Phantom Code supports 11 programming languages including Python, Java, C++, JavaScript, TypeScript, Go, Rust, Ruby, Swift, Kotlin, and C#. Set your preferred language and get idiomatic solutions.
What coding mistakes does Phantom Code help me avoid during Group By interviews?
Phantom Code catches common mistakes like off-by-one errors, missing edge cases, incorrect base conditions, and suboptimal approaches. It provides clean, well-structured code that handles all edge cases.
Is Phantom Code detectable when solving problems like Group By during live interviews?
No. Phantom Code runs as an invisible desktop overlay that doesn't appear in screen shares, recordings, or proctoring software. It works with Zoom, HackerRank, CodeSignal, CoderPad, and all major platforms.
How does Phantom Code help when I'm struggling with Group By style questions under pressure?
The AI provides structured guidance: first the approach (what algorithm/data structure to use and why), then the implementation with clean code, and finally complexity analysis. This helps you think clearly even under pressure.
Does Phantom Code work for the coding platforms used in Group By interviews?
Yes. Phantom Code works with all major coding platforms including LeetCode, HackerRank, CodeSignal, CoderPad, HackerEarth, and any browser-based coding environment.
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