// LinkedIntList provides an implementation of the IntList interface backed by
// a doubly-linked list with dummy header nodes at the front and back.  This
// will provide O(1) performance for adding at the front or back of the list
// and removing values while iterating over the list.  All of the iterator's
// methods are O(1).  Methods like get and set will be O(n) for values in the
// middle of the list.

import java.util.*;

public class LinkedIntList extends AbstractIntList {
    private ListNode front;  // first value in the list
    private ListNode back;   // last value in the list
    private int size;        // current number of elements

    // post: constructs an empty list
    public LinkedIntList() {
        front = new ListNode(0);
        back = new ListNode(0);
        clear();
    }

    // post: returns the current number of elements in the list
    public int size() {
        return size;
    }

    // pre : 0 <= index < size()
    // post: returns the integer at the given index in the list
    public int get(int index) {
        checkIndex(index);
        ListNode current = nodeAt(index);
        return current.data;
    }

    // post: appends the given value to the end of the list
    public void add(int value) {
        add(size, value);
    }

    // pre: 0 <= index <= size()
    // post: inserts the given value at the given index, shifting subsequent
    //       values right
    public void add(int index, int value) {
        if (index < 0 || index > size) {
            throw new IndexOutOfBoundsException("index: " + index);
        }
        ListNode current = nodeAt(index - 1);
        ListNode newNode = new ListNode(value, current.next, current);
        current.next = newNode;
        newNode.next.prev = newNode;
        size++;
    }

    // pre : 0 <= index < size()
    // post: replaces the integer at the given index with the given value
    public void set(int index, int value) {
        checkIndex(index);
        ListNode current = nodeAt(index);
        current.data = value;
    }

    // pre : 0 <= index < size()
    // post: removes value at the given index, shifting subsequent values left
    public void remove(int index) {
        checkIndex(index);
        ListNode current = nodeAt(index - 1);
        current.next = current.next.next;
        current.next.prev = current;
        size--;
    }

    // post: list is empty
    public void clear() {
        front.next = back;
        back.prev = front;
        size = 0;
    }

    // post: returns an iterator for this list
    public Iterator<Integer> iterator() {
        return new LinkedIterator();
    }

    // pre : 0 <= index < size()
    // post: returns the node at a specific index.  Uses the fact that the list
    //       is doubly-linked to start from the front or the back, whichever
    //       is closer.
    private ListNode nodeAt(int index) {
        ListNode current;
        if (index < size / 2) {
            current = front;
            for (int i = 0; i < index + 1; i++) {
                current = current.next;
            }
        } else {
            current = back;
            for (int i = size; i >= index + 1; i--) {
                current = current.prev;
            }
        }
        return current;
    }

    private static class ListNode {
        public int data;       // data stored in this node
        public ListNode next;  // link to next node in the list
        public ListNode prev;  // link to previous node in the list

        // post: constructs a node with given data and null links
        public ListNode(int data) {
            this(data, null, null);
        }

        // post: constructs a node with given data and given links
        public ListNode(int data, ListNode next, ListNode prev) {
            this.data = data;
            this.next = next;
            this.prev = prev;
        }
    }

    private class LinkedIterator implements Iterator<Integer> {
        private ListNode current;  // location of next value to return
        private boolean removeOK;  // whether it's okay to remove now

        // post: constructs an iterator for the given list
        public LinkedIterator() {
            current = front.next;
            removeOK = false;
        }

        // post: returns true if there are more elements left, false otherwise
        public boolean hasNext() {
            return current != back;
        }

        // pre : hasNext() (throws NoSuchElementException if not)
        // post: returns the next element in the iteration
        public Integer next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }
            int result = current.data;
            current = current.next;
            removeOK = true;
            return result;
        }

        // pre : next() has been called without a call on remove (i.e., at most
        //       one call per call on next; throws IllegalStateException if
        //       not)
        // post: removes the last element returned by the iterator
        public void remove() {
            if (!removeOK) {
                throw new IllegalStateException();
            }

            ListNode prev2 = current.prev.prev;
            prev2.next = current;
            current.prev = prev2;
            size--;
            removeOK = false;
        }
    }
}