Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.

Given 1->1->2->3->3, return 1->2->3.

思路：此题与上一题异曲同工，详细解法例如以下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {

        ListNode first = new ListNode(0);
        ListNode last = first;
        
        ListNode p = head;
        
        while(head != null){
        	while(head.next != null){//去除反复项
        		if(p.val == head.next.val){
        			head = head.next;
        		}else{
        			break;
        		}
        	}
        	last.next = p;//每项仅仅加入一个值
        	last = last.next;
        	p = head = head.next;
        	last.next = null;
        }
		return first.next;
    }
}