/**
     * 利用歸并排序計算逆序數
     * @param v 數字序列
     * @param l 左邊界
     * @param r 有邊界
     * @return 左右邊界之間的逆序數
     */
    public static int findRev_order_num(int[] v,int l,int r){
        int len = r-l;
        if(len==0){
            return 0;
        }else if(len==1){
            if(v[l]>v[r]){
                int tmp = v[l];
                v[l] = v[r];
                v[r] = tmp;
                return 1;
            }
        }else{
            int mid = l+len/2;
            int leftNum = findRev_order_num(v,l,mid);
            int rightNum = findRev_order_num(v,mid+1,r);
            int merNum = 0;
            int[] dp = new int[r-l+1];
            int index=0;
            int i=l;
            int j=mid+1;
            while(i<=mid && j<=r){
                if(v[i]>v[j]){
                    merNum+=mid-i+1;
                    dp[index++] = v[j];
                    j++;
                }else{
                    dp[index++] = v[i];
                    i++;
                }
            }
            while(i<=mid){
                dp[index++] = v[i++];
            }
            while(j<=r){
                dp[index++] = v[j++];
            }
            System.arraycopy(dp,0,v,l,dp.length);
            return (leftNum+rightNum+merNum);
        }
        return 0;
    }