題目：

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. 

演算法如下：

/**
 * 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) {
        if(head==null){
            return null;
        }
        ListNode current = head;
        ListNode pre = head;
        while(current.next!=null){
            current = current.next;
            int value = pre.val;
            while
 
(current.val == value){
                if(current.next==null){
                    pre.next=null;
                    break;
                }
                pre.next = current.next;
                current = current.next;
                if(current==null){
                    break;
                }
            }
            pre = pre.next;
        }
        return
 
 head;
    }
}