选择

Transcript

1. Selection Sort
2. Selection Sort • In selection sort, the idea of the algorithm is to find the smallest unsorted element and add it to the end of the sorted list. In pseudocode: • Repeat until no unsorted elements remain: • Search the unsorted part of the data to find the smallest value • Swap the smallest found value with the first element of the unsorted part
3. Selection Sort 5 2 1 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
4. Selection Sort 5 2 1 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
5. Selection Sort 1 2 5 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
6. Selection Sort 1 2 5 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
7. Selection Sort 1 2 5 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
8. Selection Sort 1 2 5 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
9. Selection Sort 1 2 5 3 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
10. Selection Sort 1 2 3 5 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
11. Selection Sort 1 2 3 5 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
12. Selection Sort 1 2 3 5 6 4 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
13. Selection Sort 1 2 3 4 6 5 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
14. Selection Sort 1 2 3 4 6 5 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
15. Selection Sort 1 2 3 4 6 5 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
16. Selection Sort 1 2 3 4 5 6 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
17. Selection Sort 1 2 3 4 5 6 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
18. Selection Sort 1 2 3 4 5 6 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
19. Selection Sort 1 2 3 4 5 6 In pseudocode: Repeat until no unsorted elements remain: Search the unsorted part of the data to find the smallest value Swap the smallest found value with the first element of the unsorted part
20. Selection Sort • Worst-case scenario: We have to iterate over each of the n elements of the array (to find the smallest unsorted element) and we must repeat this process n times, since only one element gets sorted on each pass. • Best-case scenario: Exactly the same! There’s no way to guarantee the array is sorted until we go through this process for all the elements.
21. Selection Sort O(n2) Ω(n2)