归并求小和

public class SmallSum {

   public static int sum=0;
        public static void main(String[] args) {
            //int[] arr={1,3,4,2,5};
            int[] arr={1,3};
            process(arr,0,arr.length-1);
            for (int i1 : arr) {
                System.out.println(i1);
            }
            System.out.println("小和为："+sum);
        }
        public static int process(int[] arr,int l,int r) {
            if (l==r) return 0;
            int mid=l+((r-l)>>1);
            return process(arr,l,mid)+
            process(arr,mid+1,r)+
            merge(arr,l,mid,r);
        }

        public static int  merge(int[] arr,int l,int mid,int r) {

            int[] help=new int[r-l+1];
            int i=0;
            int p=l;
            int q=mid+1;
            while(p<=mid&&q<=r)
            {
                sum+=arr[p]<arr[q]?(r-q+1)*arr[p]:0;
                help[i++]=arr[p]<arr[q]?arr[p++]:arr[q++];


            }
            while(p<=mid){
                help[i++]=arr[p++];
            }
            while (q<=r){
                help[i++]=arr[q++];
            }
            for (int i1 = 0; i1 < help.length; i1++) {
                arr[l+i1]=help[i1];
            }
            return sum;

    }

}