Algs4-1.4TwoSumFast

import java.util.Arrays;
public class TwoSumFast
{
    public static int count(int[] a)
    {
        Arrays.sort(a);
        int N=a.length;
        int cnt=0;
        for(int i=0;i<N;i++)
            if (BinarySearch.rank(-a[i],a)>i)
               cnt++;
        return cnt;
    }
   
    public static void main (String[] args)
    {
        int[] a=In.readInts(args[0]);
        StdOut.println(count(a));
    }
}