120 lines
4.0 KiB
Java
120 lines
4.0 KiB
Java
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/*************************************************************************
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* Compilation: javac ThreeSumFast.java
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* Execution: java ThreeSumFast input.txt
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* Dependencies: StdOut.java In.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 N^2 log N 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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*
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* Limitations
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* -----------
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* - we ignore integer overflow
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* - doesn't handle case when input has duplicates
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*
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*
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* % java ThreeSumFast 1Kints.txt
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* 70
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*
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* % java ThreeSumFast 2Kints.txt
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* 528
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*
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* % java ThreeSumFast 4Kints.txt
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* 4039
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*
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* % java ThreeSumFast 8Kints.txt
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* 32074
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*
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* % java ThreeSumFast 16Kints.txt
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* 255181
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*
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* % java ThreeSumFast 32Kints.txt
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* 2052358
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*
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*************************************************************************/
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import java.util.Arrays;
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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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* The ThreeSumFast class provides static methods for counting
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* and printing the number of triples in an array of distinct integers that
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* sum to 0 (ignoring integer overflow).
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*
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* This implementation uses sorting and binary search and takes time
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* proportional to N^2 log N, 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 ThreeSumFast {
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// returns true if the sorted array a[] contains any duplicated integers
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private static boolean containsDuplicates(int[] a) {
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for (int i = 1; i < a.length; i++)
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if (a[i] == a[i-1]) return true;
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return false;
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}
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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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* @throws IllegalArgumentException if the array contains duplicate 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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Arrays.sort(a);
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if (containsDuplicates(a)) throw new IllegalArgumentException("array contains duplicate integers");
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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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int k = Arrays.binarySearch(a, -(a[i] + a[j]));
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if (k > j) 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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* 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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Arrays.sort(a);
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if (containsDuplicates(a)) throw new IllegalArgumentException("array contains duplicate integers");
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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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int k = Arrays.binarySearch(a, -(a[i] + a[j]));
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if (k > j) cnt++;
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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 distinct 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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int cnt = count(a);
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StdOut.println(cnt);
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}
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}
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