《算法导论》Problem 2-4 Inversions

在Merge Sort的基础上改改就好了。

 public class Inversions {

     public static int inversions(int [] A,int p, int r)
     {   

         if(p<r)
         {
             int q = (int) Math.floor( (p+r)/2 );
             int left = inversions(A,p,q);
             int right = inversions(A,q+1,r);
             int c = combine(A,p,q,r);
             return left + right + c;
         }
         return 0;

     }
     public static int combine(int [] A, int p, int q, int r)
     {
         int total_inversions = 0;

         int n1 = q-p+1;
         int n2 = r-q;
         int [] L = new int[n1];
         int [] R = new int[n2];
         for(int i=0; i<n1; i++)
           L[i] = A[p+i];
         for(int i=0; i<n2;i++)
             R[i]=A[q+i+1];

         int i=0;
         int j=0;
         int counter = p;
         while(i<L.length && j<R.length)
         {
             if(L[i]<=R[j])
             {
                 A[counter]=L[i];
                 i++;
             }
             else
             {
                 A[counter]=R[j];
                 j++;
                 total_inversions += (q-(p+i)+1);
             }
             counter++;
         }
         if(i==L.length)
         {
             for(int k=j;k<R.length;k++)
                 A[counter++] = R[k];
         }
         else
         {
             for(int k=i;k<L.length;k++)
                 A[counter++] = L[k];
         }
         return total_inversions;
     }
     public static void main(String[] args) {
         int A[] = {2,3,8,6,1,7,9,1};
         int num_of_inversions = inversions(A,0,A.length-1);
         System.out.println(num_of_inversions);
     }
 }