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javaalgorithmsortingarray-algorithms

How to find the permutation of a sort in Java

I want to sort an array and find the index of each element in the sorted order. So for instance if I run this on the array:

[3,2,4]

I'd get:

[1,0,2]

Is there an easy way to do this in Java?

Solution

  • Let's assume your elements are stored in an array. 

    final int[] arr = // elements you want
List<Integer> indices = new ArrayList<Integer>(arr.length);
for (int i = 0; i < arr.length; i++) {
  indices.add(i);
}
Comparator<Integer> comparator = new Comparator<Integer>() {
  public int compare(Integer i, Integer j) {
    return Integer.compare(arr[i], arr[j]);
  }
}
Collections.sort(indices, comparator);

    Now indices contains the indices of the array, in their sorted order. You can convert that back to an int[] with a straightforward enough for loop.