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package algs14;
import stdlib.*;
import java.util.Arrays;
/* ***********************************************************************
 *  Compilation:  javac ThreeSumFast.java
 *  Execution:    java ThreeSumFast input.txt
 *  Data files:   http://algs4.cs.princeton.edu/14analysis/1Kints.txt
 *                http://algs4.cs.princeton.edu/14analysis/2Kints.txt
 *                http://algs4.cs.princeton.edu/14analysis/4Kints.txt
 *                http://algs4.cs.princeton.edu/14analysis/8Kints.txt
 *                http://algs4.cs.princeton.edu/14analysis/16Kints.txt
 *                http://algs4.cs.princeton.edu/14analysis/32Kints.txt
 *                http://algs4.cs.princeton.edu/14analysis/1Mints.txt
 *
 *  A program with N^2 log N running time. Read in N integers
 *  and counts the number of triples that sum to exactly 0.
 *
 *  Limitations
 *  -----------
 *     - we ignore integer overflow
 *     - doesn't handle case when input has duplicates
 *
 *
 *  % java ThreeSumFast 1Kints.txt
 *  70
 *
 *  % java ThreeSumFast 2Kints.txt
 *  528
 *
 *  % java ThreeSumFast 4Kints.txt
 *  4039
 *
 *  % java ThreeSumFast 8Kints.txt
 *  32074
 *
 *  % java ThreeSumFast 16Kints.txt
 *  255181
 *
 *  % java ThreeSumFast 32Kints.txt
 *  2052358
 *
 *************************************************************************/

public class ThreeSumFast {

  // print distinct triples (i, j, k) such that a[i] + a[j] + a[k] = 0
  public static void printAll(int[] a) {
    int N = a.length;
    Arrays.sort(a);
    for (int i = 0; i < N; i++) {
      for (int j = i+1; j < N; j++) {
        int k = Arrays.binarySearch(a, -(a[i] + a[j]));
        if (k > j) StdOut.println(a[i] + " " + a[j] + " " + a[k]);
      }
    }
  }

  // return number of distinct triples (i, j, k) such that a[i] + a[j] + a[k] = 0
  public static int count(int[] a) {
    int N = a.length;
    Arrays.sort(a);
    int cnt = 0;
    for (int i = 0; i < N; i++) {
      for (int j = i+1; j < N; j++) {
        int k = Arrays.binarySearch(a, -(a[i] + a[j]));
        if (k > j) cnt++;
      }
    }
    return cnt;
  }

  public static void main(String[] args)  {
    //args = new String[] { "data/2Kints.txt" };

    int[] a = new In(args[0]).readAllInts();
    int cnt = count(a);
    StdOut.println(cnt);
  }
}