What I meant is something like this. I am unsure if its better, considering the tradeoff is having to check some if statements just to avoid creating empty Buffers. I am unsure what the "price" for creating empty buffers is.. I am pretty sure this would be better for a large scale with lots of empty Buffers, but this is obviously not the case here.

public static void main(String[] args) {

Scanner scan=new Scanner(System.in);

int size=Integer.parseInt(scan.nextLine());

StringBuffer[] st=new StringBuffer[100]; 

for(int i=0;i<size;i++)
{
    String sts=scan.nextLine();
    String[] str=sts.split("[\\s]+");
    int k=Integer.parseInt(str[0]);
    String s;
    if(i<size/2)
        s="- ";
    else
        s=str[1]+" ";
    if(st[k] == null) st[k] = new StringBuffer();
    st[k].append(s);
}

for(int i=0;i<100;i++)
{
    if(st[i] != null) System.out.print(st[i]);
}

}