使用比较器无法根据另一个数组对一个数组进行排序

Question

I want to sort one array based on another array in java. I can do using Pair data structure but using directly another array in comparator not working.

/* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
                  final int[] l = new int[]{9, 17, 39, 35, 20, 18, 34, 11, 2, 45, 46, 15, 33, 47, 10, 27};
     final List<Integer> n = Arrays.asList(32, 39, 86, 81, 64, 53, 76, 40, 46, 63, 88, 56, 52, 50, 22, 38);
    Collections.sort(n,  new Comparator<Integer>() {
    public int compare(Integer left, Integer right) {
        //System.out.println(n.indexOf(left) + " "+n.indexOf(right));
        return l[n.indexOf(left)]-l[n.indexOf(right)];
    }
});
    //Collections.sort(l);
    System.out.println(n);
    System.out.println(l);
    }
}

Getting output - [50, 88, 63, 81, 38, 86, 39, 32, 76, 56, 64, 53, 40, 52, 46, 22] Expected output - [46,32 and so on ] corresponding to [2,9 and so on]

Answer 1

Your problem is that the reference order changes as you sort the list. You start with 9->32 and 17->39 for the first two elements. After comparing and changing the first two elements the "mapping" could change and you would have 9->39 and 17->32 instead for the next iteration.

To solve that you need to create a copy of the list. Then, while sorting n you refer to that copy for the order. Something like this should work:

public static void main (String[] args) throws java.lang.Exception
{
  final int[] l = new int[]{9, 17, 39, 35, 20, 18, 34, 11, 2, 45, 46, 15, 33, 47, 10, 27};
  final List<Integer> n = Arrays.asList(32, 39, 86, 81, 64, 53, 76, 40, 46, 63, 88, 56, 52, 50, 22, 38);
  List<Integer> listCopy = new ArrayList(n);
  Collections.sort(n,  new Comparator<Integer>() {
    public int compare(Integer left, Integer right) {
        return l[listCopy.indexOf(left)]-l[listCopy.indexOf(right)]; \\notice the listCopy.indexOf()
    }
  });
  System.out.println(n);
}