143. Reorder List

Given a singly linked list L: L0→L1→…→Ln-1→Ln, reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…

You must do this in-place without altering the nodes' values.

For example, Given {1,2,3,4}, reorder it to {1,4,2,3}.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        if(head == null) return ;
        ListNode p = head;
        ListNode q = head.next;
        while(q != null && q.next != null){
            p = p.next;
            q = q.next;
            if(q != null) q = q.next;
        }

        q = p.next;
        p.next = null;
        ListNode newHead = null;
        while(q != null){
            ListNode tmp = q.next;
            q.next = newHead;
            newHead = q;
            q = tmp;
        }

        p = head;
        q = newHead;
        ListNode dummy = new ListNode(-1);
        ListNode tail = dummy;

        while(p != null || q != null){
            if(p != null){
                tail.next = p;
                p = p.next;
                tail = tail.next;
            }
            if(q!= null){
                tail.next = q;
                q = q.next;
                tail = tail.next;
            }

        }

        head = dummy.next;

    }
}