Repeating number problem - 1

Here is the collection of problems that related to finding  the duplicate numbers in a array.

1. Given an array of numbers ranging from 1 to N, in which numbers could repeat itself any number of times, find the duplicate numbers in O(n) with constant memory space.

Method 1: Use an extra array keep track of the elements counting.

public class Duplicate {
    public static void main(String args[]) {
        int arr[] = {1, 3, 5, 8, 5, 7, 4, 2, 4};
        findDuplicate(arr);

    }
    public static void findDuplicate(int arr[]) {
        // Using the current number as the key/index to the value
        for(int i = 0; i < arr.length; i++) {
            // Positvie - never seen before
            if( arr[Math.abs(arr[i]) - 1] > 0 )  {
                arr[Math.abs(arr[i]) - 1] =  -arr[Math.abs(arr[i]) - 1];
            }
            // Negative - already seen, duplicate 
            else if(arr[Math.abs(arr[i]) - 1] < 0) {
                System.out.print(Math.abs(arr[i]) + " ");
                arr[Math.abs(arr[i]) - 1] = 0;
            }
            else {// 0 - already detected
            }
        }
        System.out.println();
    }
}

output:

5 4