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Sorting (introduction)

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Sorting (introduction)

  1. 1. Sorting Arvind Devaraj
  2. 2. Sorting • Given an array, put the elements in order – Numerical or lexicographic • Desirable characteristics – Fast – In place (don’t need a second array) – Stability
  3. 3. Insertion Sort • Simple, able to handle any data • Grow a sorted array from the beginning – Create an empty array of the proper size – Pick the elements one at a time in any order – Put them in the new array in sorted order • If the element is not last, make room for it – Repeat until done • Can be done in place if well designed
  4. 4. Insertion Sort 90 11 27 37111631 4
  5. 5. Insertion Sort 90 11 27 37111631 4 90
  6. 6. Insertion Sort 90 11 27 37111631 4 9011
  7. 7. Insertion Sort 90 11 27 90 37111631 4 2711
  8. 8. Insertion Sort 90 11 27 31 90 37111631 4 2711
  9. 9. Insertion Sort 90 11 27 27 31 90 37111631 4 114
  10. 10. Insertion Sort 90 11 27 16 27 31 90 37111631 4 114
  11. 11. Insertion Sort 90 11 27 11 16 27 31 90 37111631 4 114
  12. 12. Insertion Sort 90 11 27 11 16 27 31 37 90 37111631 4 114
  13. 13. Merge Sort • Fast, able to handle any data – But can’t be done in place • View the array as a set of small sorted arrays – Initially only the 1-element “arrays” are sorted • Merge pairs of sorted arrays – Repeatedly choose the smallest element in each – This produces sorted arrays that are twice as long • Repeat until only one array remains
  14. 14. Merge Sort 90 11 27 37111631 4 9011
  15. 15. Merge Sort 90 11 27 27 31 37111631 4 9011
  16. 16. Merge Sort 90 11 27 27 31 4 16 37111631 4 9011
  17. 17. Merge Sort 90 11 27 27 31 4 16 11 37 37111631 4 9011
  18. 18. Merge Sort 11 27 31 27 31 4 16 11 37 90 9011
  19. 19. Merge Sort 11 27 31 27 31 4 16 11 37 37161190 4 9011
  20. 20. Merge Sort 11 27 31 11 16 27 31 37 90 37161190 4 114
  21. 21. Divide and Conquer • Split a problem into simpler subproblems – Keep doing that until trivial subproblems result • Solve the trivial subproblems • Combine the results to solve a larger problem – Keep doing that until the full problem is solved • Merge sort illustrates divide and conquer – But it is a general strategy that is often helpful
  22. 22. Quick Sort • For example, given 80 38 95 84 99 10 79 44 26 87 96 12 43 81 3 we can select the middle entry, 44, and sort the remaining entries into two groups, those less than 44 and those greater than 44: 38 10 26 12 43 3 44 80 95 84 99 79 87 96 81 • If we sort each sub-list, we will have sorted the entire array
  23. 23. A sample heap • Each node is larger than its children 19 1418 22 321 14 119 15 25 1722
  24. 24. Sorting using Heaps • What do heaps have to do with sorting an array? Because the binary tree is balanced and left justified, it can be represented as an array – All our operations on binary trees can be represented as operations on arrays – To sort: heapify the array; while the array isn’t empty { remove and replace the root; reheap the new root node; }
  25. 25. Summary of Sorting Algorithms • in-place, randomized • fastest (good for large inputs) O(n log n) expected quick-sort • sequential data access • fast (good for huge inputs) O(n log n)merge-sort • in-place • fast (good for large inputs) O(n log n)heap-sort O(n2) O(n2) Time insertion-sort selection-sort Algorithm Notes • in-place • slow (good for small inputs) • in-place • slow (good for small inputs)

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