逆序对的数量（归并排序求解）

import java.util.Scanner;

public class Main {
         static long cnt=0;
         static int b[]=new int[100005];
         static int a[]=new int[100005];
         public static void merge_sort(int a[],int l,int r){
               if(l>=r) return;
               int mid=l+r>>1;
               merge_sort(a,l,mid);
               merge_sort(a,mid+1,r);
               int k=l,i=l,j=mid+1;
               while(i<=mid&&j<=r){
                     if(a[i]>a[j]){
                           b[k++]=a[j++];
                           cnt+=(mid-i+1);
                     }
                     else b[k++]=a[i++];
               }
               while(i<=mid) b[k++]=a[i++];
               while(j<=r) b[k++]=a[j++];
               for(int t=l;t<=r;t++) a[t]=b[t];
         }
         public static void main(String[] args) {
              Scanner scan=new Scanner(System.in);
              int n=scan.nextInt();
              for(int i=0;i<n;i++) a[i]=scan.nextInt();
              merge_sort(a,0,n-1);
              System.out.println(cnt);
        }
}