Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

这道题需要用一个dummy，然后fast和slow指针去做。

/**
 * 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 dummy = new ListNode(0);
        ListNode slow = dummy;
        ListNode fast = head;
        slow.next = fast;
        while(fast != null){
            while(fast.next != null && fast.val == fast.next.val){
                fast = fast.next;
            }
            if(slow.next == fast){
                slow = slow.next;
                fast = fast.next;
            }
            else{
                slow.next = fast.next;
                fast = fast.next;
            }
            
        }
        return dummy.next;
    }
}