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101 lines
3.3 KiB
Java
101 lines
3.3 KiB
Java
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import edu.princeton.cs.introcs.In;
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import edu.princeton.cs.introcs.StdOut;
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/*************************************************************************
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* Compilation: javac ThreeSum.java
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* Execution: java ThreeSum input.txt
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* Dependencies: In.java StdOut.java Stopwatch.java
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* Data files: http://algs4.cs.princeton.edu/14analysis/1Kints.txt
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* http://algs4.cs.princeton.edu/14analysis/2Kints.txt
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* http://algs4.cs.princeton.edu/14analysis/4Kints.txt
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* http://algs4.cs.princeton.edu/14analysis/8Kints.txt
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* http://algs4.cs.princeton.edu/14analysis/16Kints.txt
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* http://algs4.cs.princeton.edu/14analysis/32Kints.txt
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* http://algs4.cs.princeton.edu/14analysis/1Mints.txt
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*
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* A program with cubic running time. Read in N integers
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* and counts the number of triples that sum to exactly 0
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* (ignoring integer overflow).
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*
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* % java ThreeSum 1Kints.txt
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* 70
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*
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* % java ThreeSum 2Kints.txt
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* 528
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*
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* % java ThreeSum 4Kints.txt
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* 4039
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*
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*************************************************************************/
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/**
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* The ThreeSum class provides static methods for counting
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* and printing the number of triples in an array of integers that sum to 0
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* (ignoring integer overflow).
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*
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* This implementation uses a triply nested loop and takes proportional to N^3,
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* where N is the number of integers.
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*
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* For additional documentation, see <a href="http://algs4.cs.princeton.edu/14analysis">Section 1.4</a> of
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* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
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*
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* @author Robert Sedgewick
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* @author Kevin Wayne
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*/
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public class ThreeSum {
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/**
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* Prints to standard output the (i, j, k) with i < j < k such that a[i] + a[j] + a[k] == 0.
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* @param a the array of integers
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*/
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public static void printAll(int[] a) {
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int N = a.length;
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for (int i = 0; i < N; i++) {
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for (int j = i+1; j < N; j++) {
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for (int k = j+1; k < N; k++) {
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if (a[i] + a[j] + a[k] == 0) {
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StdOut.println(a[i] + " " + a[j] + " " + a[k]);
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}
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}
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}
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}
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}
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/**
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* Returns the number of triples (i, j, k) with i < j < k such that a[i] + a[j] + a[k] == 0.
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* @param a the array of integers
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* @return the number of triples (i, j, k) with i < j < k such that a[i] + a[j] + a[k] == 0
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*/
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public static int count(int[] a) {
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int N = a.length;
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int cnt = 0;
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for (int i = 0; i < N; i++) {
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for (int j = i+1; j < N; j++) {
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for (int k = j+1; k < N; k++) {
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if (a[i] + a[j] + a[k] == 0) {
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cnt++;
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}
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}
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}
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}
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return cnt;
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}
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/**
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* Reads in a sequence of integers from a file, specified as a command-line argument;
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* counts the number of triples sum to exactly zero; prints out the time to perform
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* the computation.
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*/
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public static void main(String[] args) {
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In in = new In(args[0]);
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int[] a = in.readAllInts();
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Stopwatch timer = new Stopwatch();
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int cnt = count(a);
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StdOut.println("elapsed time = " + timer.elapsedTime());
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StdOut.println(cnt);
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}
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}
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