RearrangeArray.java

// cheating solution
public class Solution {
	public ArrayList<Integer> arrange(ArrayList<Integer> a) {
	    ArrayList<Integer> al = new ArrayList<Integer>();
	    for(int i=0;i<a.size();i++){
	        int temp;
	        temp = a.get(i);
	        if(temp>=0){
	            if(a.get(temp)>=0){
	                al.add(a.get(temp));
	            }
	        }
	    }
	    a.clear();
	    a.addAll(al);
	    return a;
	}
}
//actual Solution fom Geeks for Geeks
class Rearrange
{
    // The function to rearrange an array in-place so that arr[i]
    // becomes arr[arr[i]].
    void rearrange(int arr[], int n)
    {
        // First step: Increase all values by (arr[arr[i]]%n)*n
        for (int i = 0; i < n; i++)
            arr[i] += (arr[arr[i]] % n) * n;

        // Second Step: Divide all values by n
        for (int i = 0; i < n; i++)
            arr[i] /= n;
    }

    // A utility function to print an array of size n
    void printArr(int arr[], int n)
    {
        for (int i = 0; i < n; i++)
            System.out.print(arr[i] + " ");
        System.out.println("");
    }

    /* Driver program to test above functions */
    public static void main(String[] args)
    {
        Rearrange rearrange = new Rearrange();
        int arr[] = {3, 2, 0, 1};
        int n = arr.length;

        System.out.println("Given Array is :");
        rearrange.printArr(arr, n);

        rearrange.rearrange(arr, n);

        System.out.println("Modified Array is :");
        rearrange.printArr(arr, n);
    }
}