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CS 112 - Fall 2012 , Lab 03

CS 112 - Fall 2012 , Lab 03. Haohan Zhu. Random Class. http://docs.oracle.com/javase/7/docs/api/java/util/Random.html Create an object for Random() Random r = new Random();

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CS 112 - Fall 2012 , Lab 03

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  1. CS 112 - Fall 2012, Lab 03 Haohan Zhu

  2. CS 112 - Fall 2012, Lab 03 Random Class • http://docs.oracle.com/javase/7/docs/api/java/util/Random.html • Create an object for Random() • Random r = new Random(); • Use method .nextInt(n) to generate a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value n (exclusive)

  3. CS 112 - Fall 2012, Lab 03 Evaluation by Time • http://docs.oracle.com/javase/7/docs/api/java/lang/System.html • System.nanoTime(); • System.currentTimeMillis();

  4. CS 112 - Fall 2012, Lab 03 Practice: Inversion • Let int A[] be an array of distinct integers. There are len = A.length distinct integers. • If i<j and A[i]>A[j], then the pair (i, j) is called an inversion of A. • There exists many such pairs in A. Compute how many inversions in A.

  5. CS 112 - Fall 2012, Lab 03 Practice: Inversion • Intuitive method: O(n^2) 1 3 2 5 7 4 6 8 1 3 2 5 7 4 6 8 1 3 2 5 7 4 6 8 1 3 2 5 7 4 6 8

  6. CS 112 - Fall 2012, Lab 03 Practice: Inversion • Maximal number of inversions ? • Find an efficient algorithm • When the array is partly sorted, to determine how many inversions are caused by a number, we do not need to compare every pair of numbers. For instance, in array {1,3,5,7,2,4,6,8}, consider the number of inversions caused by 2.

  7. CS 112 - Fall 2012, Lab 03 Practice: Inversion • Use the process similar to merge sort 1 3 2 5 7 4 6 8 1 3 2 5 7 4 6 8 1 3 2 5 7 4 6 8 1 3 2 5 7 4 6 8

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