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

Example 1:

Input: 1->2->3->3->4->4->5
Output: 1->2->5

Example 2:

Input: 1->1->1->2->3
Output: 2->3

难度：medium

题目：给出一排序链表，删除所有重复结点，只保留原链表中不重复的结点。

思路：双指针遍历

Runtime: 0 ms, faster than 100.00% of Java online submissions for Remove Duplicates from Sorted List II.
Memory Usage: 37.8 MB, less than 0.94% of Java online submissions for Remove Duplicates from Sorted List II.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if (null == head) {
            return head;
        }
        
        ListNode dummyHead = new ListNode(0);
        dummyHead.next = head;
        ListNode prev = dummyHead, ptr = head;
        while (ptr != null && ptr.next != null) {
            if (ptr.val == ptr.next.val) {
                while (ptr.next != null && ptr.val == ptr.next.val) {
                    ptr = ptr.next;
                }
                prev.next = ptr.next;
                ptr = prev;
            } 
            prev = ptr;
            ptr = ptr.next;
        }
        
        return dummyHead.next;
    }
}