``` 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 ``` ```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); } } ```