Created
March 16, 2016 07:06
-
-
Save baljanak/bddb000025be754b2860 to your computer and use it in GitHub Desktop.
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| #!/usr/env python | |
| def merge_and_count_split_inversions(A, B, C): | |
| """ O(n)""" | |
| i = 0 | |
| j = 0 | |
| inv = 0 | |
| for k in range(len(A) + len(B)): | |
| # If left node items are done, just fill up remaining right node items | |
| if i == len(A): | |
| C[k] = B[j] | |
| j += 1 | |
| continue | |
| # If right node items are done, just fill up remaining left node items | |
| if j == len(B): | |
| C[k] = A[i] | |
| i += 1 | |
| continue | |
| # Compare items in left and right node | |
| if A[i] < B[j]: | |
| C[k] = A[i] | |
| i += 1 | |
| else: | |
| C[k] = B[j] | |
| j += 1 | |
| # Number of elements left in first array are split inversions | |
| inv += len(A[i:]) | |
| return inv | |
| def merge_sort_with_inversions(l): | |
| """ Uses Divide and Conquer method to sort a list and count inversions""" | |
| # Base case | |
| if len(l) == 1: | |
| return 0 | |
| # split into smaller lists and call recursively - Divide step | |
| left = l[:len(l)//2] | |
| right = l[len(l)//2:] | |
| # note - 'left' and 'right' list variables are *necessary*, | |
| # they store the updated list (l) when merge is called. | |
| x = merge_sort_with_inversions(left) | |
| y = merge_sort_with_inversions(right) | |
| # merge and update the list with the merge result - Conquer ste | |
| z = merge_and_count_split_inversions(left, right, l) | |
| # print "Merge of {} and {} gives {}".format(left, right, l) | |
| return x + y + z | |
| def main(): | |
| ilist = [4,5,6,2,1,8,9,0] | |
| #ilist = [1,3,5,2,4,6] | |
| with open('IntegerArray.txt') as fd: | |
| inlist = fd.read().split('\r\n') | |
| dc = inlist.pop() | |
| ilist = [int(i) for i in inlist] | |
| print ilist | |
| inversions = merge_sort_with_inversions(ilist) | |
| print ilist | |
| print inversions | |
| if __name__ == "__main__": | |
| main() |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment