ThreeSum.java
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/******************************************************************************
* Compilation: javac ThreeSum.java
* Execution: java ThreeSum input.txt
* Dependencies: In.java StdOut.java Stopwatch.java
* 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 cubic running time. Reads n integers
* and counts the number of triples that sum to exactly 0
* (ignoring integer overflow).
*
* % java ThreeSum 1Kints.txt
* 70
*
* % java ThreeSum 2Kints.txt
* 528
*
* % java ThreeSum 4Kints.txt
* 4039
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code ThreeSum} class provides static methods for counting
* and printing the number of triples in an array of integers that sum to 0
* (ignoring integer overflow).
* <p>
* This implementation uses a triply nested loop and takes proportional to n^3,
* where n is the number of integers.
* <p>
* For additional documentation, see <a href="http://algs4.cs.princeton.edu/14analysis">Section 1.4</a> of
* <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class ThreeSum {
// Do not instantiate.
private ThreeSum() { }
/**
* Prints to standard output the (i, j, k) with {@code i < j < k}
* such that {@code a[i] + a[j] + a[k] == 0}.
*
* @param a the array of integers
*/
public static void printAll(int[] a) {
int n = a.length;
for (int i = 0; i < n; i++) {
for (int j = i+1; j < n; j++) {
for (int k = j+1; k < n; k++) {
if (a[i] + a[j] + a[k] == 0) {
StdOut.println(a[i] + " " + a[j] + " " + a[k]);
}
}
}
}
}
/**
* Returns the number of triples (i, j, k) with {@code i < j < k}
* such that {@code a[i] + a[j] + a[k] == 0}.
*
* @param a the array of integers
* @return the number of triples (i, j, k) with {@code i < j < k}
* such that {@code a[i] + a[j] + a[k] == 0}
*/
public static int count(int[] a) {
int n = a.length;
int count = 0;
for (int i = 0; i < n; i++) {
for (int j = i+1; j < n; j++) {
for (int k = j+1; k < n; k++) {
if (a[i] + a[j] + a[k] == 0) {
count++;
}
}
}
}
return count;
}
/**
* Reads in a sequence of integers from a file, specified as a command-line argument;
* counts the number of triples sum to exactly zero; prints out the time to perform
* the computation.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
In in = new In(args[0]);
int[] a = in.readAllInts();
Stopwatch timer = new Stopwatch();
int count = count(a);
StdOut.println("elapsed time = " + timer.elapsedTime());
StdOut.println(count);
}
}
/******************************************************************************
* Copyright 2002-2016, Robert Sedgewick and Kevin Wayne.
*
* This file is part of algs4.jar, which accompanies the textbook
*
* Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
* Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
* http://algs4.cs.princeton.edu
*
*
* algs4.jar is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* algs4.jar is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with algs4.jar. If not, see http://www.gnu.org/licenses.
******************************************************************************/