Sorting elements of an array by frequency
Detailed guide and Python implementation for the 'Sorting elements of an array by frequency' problem.
1. Concept Overview
The 'Sorting elements of an array by frequency' problem is a key challenge in the Arrays section.
This implementation focuses on easy-level logic in Python.
We prioritize technical accuracy and code readability in our provided solutions.
2. Real-World Applications
3. Visual Intuition
Visualizing the logic flow for Sorting elements of an array by frequency.
4. Prerequisites
5. Step-by-Step Thinking
1. Understand the problem
Read the problem statement for Sorting elements of an array by frequency carefully.
2. Formulate brute force
Draft a simple iterative solution.
3. Identify inefficiency
Look for redundant calculations.
4. Optimize search path
Use hashing or sorting to speed up the process.
5. Final Implementation
Clean up the code for production standards.
Problem Statement
Write a function sort_by_frequency(arr) that sorts elements of the array by their frequency in descending order. If two elements have the same frequency, the smaller element comes first. Return the sorted array as a list.
- •1 <= len(arr) <= 10^5
- •-10^6 <= arr[i] <= 10^6
Examples
arr = [1, 1, 2, 2, 2, 3]
[2, 2, 2, 1, 1, 3]
2 appears 3 times (most frequent), 1 appears 2 times, 3 appears 1 time.
arr = [4, 4, 5, 5, 6]
[4, 4, 5, 5, 6]
4 and 5 both appear 2 times; 4 < 5 so 4 comes first. 6 appears once.
arr = [9, 9, 9]
[9, 9, 9]
Only one distinct element.
Need a Hint?
Edge Cases to Watch
- Empty input structures
- Single element inputs
- Large numerical bounds
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Interview Insights & Variations
Complexity Analysis Breakdown
Why Time: Directly evaluates all possibilities.
Why Space: Uses standard local memory.
Why Time: Optimized paths reduce total operations.
Why Space: May trade memory for speed.
Optimized Solution Python Code
Optimized Solution Python Code
from collections import Counter
def sort_by_frequency_opt(arr):
counts = Counter(arr)
# Sort by frequency descending, then by value ascending
arr.sort(key=lambda x: (-counts[x], x))
return arrBrute Force Code (Spoiler Guarded)
Brute Force Code (Spoiler Guarded)
def sort_by_frequency_brute(arr):
freq = {}
for num in arr:
freq[num] = freq.get(num, 0) + 1
# Sort by frequency (desc) and then by value (asc)
def sort_key(x):
return (-freq[x], x)
arr.sort(key=sort_key)
return arrAlgorithm Pattern Checklist
When dealing with Arrays data patterns.
Core Prerequisites
Revision Key Notes
Common Mistakes & Pitfalls
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