Given an array of integers where each value 1 <= x <= len(array), write a function that finds all the duplicates in the array. - Java Algorithm

Java examples for Algorithm:Array

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Description

Given an array of integers where each value 1 <= x <= len(array), write a function that finds all the duplicates in the array.

Demo Code

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class FindDuplicates {

    // Return a list of duplicates in the array. To avoid using extra space,
    // we flag which elements we've seen before by negating the value at
    // indexed at that value in the array.
    public static List<Integer> findDuplicates(int[] arr) {
        // Use a set for results to avoid duplicates
        Set<Integer> resultSet = new HashSet<Integer>();

        for (int i = 0; i < arr.length; i++) {
            // Translate the value into an index (1 <= x <= len(arr))
            int index = Math.abs(arr[i]) - 1;

            // If the value at that index is negative, then we've already seen
            // that value so it's a duplicate. Otherwise, negate the value at
            // that index so we know we've seen it
            if (arr[index] < 0) {
                resultSet.add(Math.abs(arr[i]));
            } else {
                arr[index] = -arr[index];
            }//from ww w.j  a  va2 s .c  o m
        }

        // Return the array to it's original state
        for (int i = 0; i < arr.length; i++) {
            arr[i] = Math.abs(arr[i]);
        }

        return new ArrayList(resultSet);
    }

    // Sample test cases
    public static void main(String[] args) {
        assert compareArrayAndListValues(findDuplicates(new int[] { 1 }),
                new int[] {}) : "Length 1, no duplicates";
        assert compareArrayAndListValues(
                findDuplicates(new int[] { 1, 2 }), new int[] {}) : "Length 2, no duplicates";
    }
    // Compare the values in an integer list with the values in an int array
    private static boolean compareArrayAndListValues(List<Integer> l,
            int[] a) {
        Arrays.sort(a);
        Collections.sort(l);

        if (l.size() != a.length)
            return false;
        for (int i = 0; i < l.size(); i++) {
            if (l.get(i) != a[i])
                return false;
        }

        return true;
    }
}

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