Sorting Algorithms¶

Overview¶

Visualizations

Due to the complexity of the sorting algorithms. Visualizations may greatly enhance the learning process. Below are two useful websites providing visualizations of common algorithms and data structures.
- visualgo.net
- Data Structure Visualization
Categories

All algorithms not mentioned in the book is for your information only. Only algorithm with bold names are covered in the book.
- Comparison based
  - Selection
  - Insertion
  - Bubble
  - Shell
  - Quick
  - Merge
  - Heap
- Non-comparison based
  - Bucket
  - Radix
  - Counting
- Divide-and-conquer \(\Theta(n \log n)\)
  - Quick
  - Merge
- simple/naive \(\Theta(n^{2})\)
  - Selection
  - Insertion
  - Bubble
- Hybrid
  - Best in real-world scenarios
  - Adopted in many C++ standard library implementations
  - Tim sort: combination of merge sort and insertion sort
  - Intro sort: combination of quick sort, heap sort, and insertion sort
- Stable sorting algorithm
  
  The order of duplicate elements is preserved after sorting.
  - Insertion
  - Merge
  - Radix
Time complexity
- simple \(\Theta(n^{2})\): insert, selection, bubble
- fast \(\Theta(n \log n)\): quick, merge, heap
- linear: radix \(\Theta(nk) = \Theta(n)\), bucket \(\Theta(n + k) = \Theta(n)\)
Space complexity

The following discussion is on array only.
- in-place (swap based) \(\Theta(1)\) for swapping
  - Iterative (loop based) \(\Theta(1)\) for looping
    - selection sort
    - bubble sort
    - insertion sort
    - shell sort
    - heap sort
  - Divide-and-conquer (recursive)
    - quick sort: \(\Theta(\log n)\) to \(\Theta(n)\) for recursion
- not-in-place (out-of-place) \(\Theta(n)\) for extra arrays
  - merge sort
  - bucket sort
  - radix sort

Array vs Linked-list¶

Sorting an array
- Array property
  - fast access by index
  - slow insertion and deletion of elements except at the end
- With the same time complexity, consider data movement overhead
- Preferred general purpose sorting algorithms
  - Quick sort
  - Merge sort
  - Heap sort
  - Shell sort
Sorting a linked-list
- Linked-list property
  - slow access by index
  - fast reconstruction of new list
  - fast access to neighboring nodes
- With the same time complexity, consider data movement overhead
- Preferred general purpose sorting algorithms
  - Merge sort
  - Insertion sort
- Time Complexity
  - \(\Theta(n \log n)\) for merge sort
  - \(\Theta(n^2)\) for insertion sort
- Space Complexity depends on the implementation

Real-World Choices¶

This section is OPTIONAL!
In real-world scenarios, there are more considerations.
- Data size is not always large
- Performance boost from cache is often significant
- We can always make more complicated sorting algorithms
For small arrays
- Insertion sort
  - Fast
  - Simple
  - Stable (original order of equal elements is preserved)
For large arrays
- Quick sort > Merge sort > Heap sort
  - Quick sort is faster than merge sort and heap sort in practice, inconsistent complexity.
  - Merge sort is stable, has consistent complexity, but requires extra space
  - Heap sort is in-place but is not stable and cannot make good use of cache
- In parallel environment
  - Merge sort
  - Quick sort
The overall winner for array is
- Hybrid
  - Use divide-and-conquer to divide the array into small subarrays and then use insertion sort to sort the small subarrays
  - Quick sort + Insertion sort
  - Merge sort + Insertion sort
For linked list
- Out-of-place algorithms are no longer requiring \(\Theta(n)\) space
- Cache is not helping at all
- Direct sort
  - Merge sort for large linked list (can be parallelized)
  - Insertion sort for small linked list
- Sort as array
  - Make an array from the linked list
  - Sort the array
  - Create the new linked list according to the sorted array
- Sort as array of pointers
  - Make an array of pointers
  - Sort the array of pointers
  - Generate the new linked list according to the sorted array