改组LinkedList时发生ArrayIndexOutOfBoundsException

Question

Okay, so I have an assignment and it goes like this:

Assume you have as input a singly-linked list containing N items (an instance of a LinkedList class in Java). You should rearrange the items in the LinkedList uniformly at random. Your algorithm should consume a logarithmic (or constant) amount of extra memory and run in time proportional to NlogN in the worst case.

For the Algorithm I was going for mergesort which when it merges does so in a random way. But I have gotten stuck with my program as it throws an ArrayIndexOutOfBoundsException but it's not in my code but in LinkedList.java

I'm not very familiar with LinkedList so I have no idea what to do, I'm not even sure I'm doing this in the right way.

The code:

import java.util.LinkedList;

public class Q02 {

public static void main(String[] args) {

    LinkedList<Integer> list = new LinkedList<>();
    list.add(1);
    list.add(2);
    list.add(3);
    list.add(4);
    list.add(5);
    list.add(6);
    list.add(7);
    list.add(8);
    list.add(9);
    list.add(0);
    System.out.println("Shuffled list: "+shuffle(list));
}

public static LinkedList shuffle(LinkedList list){
    int node = 10; //just a random node
    if(list.size()<=1){
        return list;
    }
    LinkedList<Integer>list1 = new LinkedList<>();
    LinkedList<Integer>list2 = new LinkedList<>();

    while(!list.isEmpty()){
        list1.add((Integer) list.removeFirst());
        if(!list.isEmpty()){
            list2.add((Integer) list.removeFirst());
        }
    }
    shuffle(list1);
    shuffle(list2);

    if(list2.size() < list1.size()){
        int i = (int)(Math.random() * list2.size());
        list2.set(i, node);
    }

    while(!list1.isEmpty()&&!list2.isEmpty()){
        int rand = (int)(Math.round(Math.random()));
        if(rand == 1){
            list.add(list1.removeFirst());
        }
        else if(rand == 0){
            list.add(list2.removeFirst());
        }
    }
    //If any of list1 or list2 are still empty add everything to list
    if(!list1.isEmpty()){
        list.add(list2.clone());
    }
    if(!list2.isEmpty()){
        list.add(list1.clone());
    }
    list.remove(node);
    return list;
}

}

Here is the error:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 10, Size: 2
at java.util.LinkedList.checkElementIndex(LinkedList.java:553)
at java.util.LinkedList.remove(LinkedList.java:523)
at kth.id2010.lab.lab03.Q02.Q02.shuffle(Q02.java:62)
at kth.id2010.lab.lab03.Q02.Q02.shuffle(Q02.java:39)
at kth.id2010.lab.lab03.Q02.Q02.shuffle(Q02.java:39)
at kth.id2010.lab.lab03.Q02.Q02.shuffle(Q02.java:39)
at kth.id2010.lab.lab03.Q02.Q02.main(Q02.java:22)

Answer 1

If you are getting an ArrayIndexOutOfBoundsException , that means that somewhere you are trying to access an item in the LinkedList using an index that is invalid. For example, if your LinkedList has 10 items, then listOfSize10.set(10, node) would raise an ArrayIndexOutOfBoundsException .

Most likely the error you are seeing stems from these lines:

int i = (int)(Math.random() * list2.size());
list2.set(i, node);

Can you guarantee that i is always greater than or equal to 0 and less than list2.size() ?

Update After seeing your stack trace, it looks like the error comes from these lines:

int node = 10; //just a random node
list.remove(node);

Here you are clearly trying to remove the item at index 10 (or, the eleventh item) when there are only 10 items in the list. So your array index is out of bounds.