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

Example 1:
Input: 1->1->2
Output: 1->2

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

难度：easy

题目：
给定有序列表，删除所有重复的元素只允许其出现一次。

Runtime: 1 ms, faster than 29.69% of Java online submissions for Remove Duplicates from Sorted List.
Memory Usage: 27.4 MB, less than 6.14% of Java online submissions for Remove Duplicates from Sorted List.

/**
 * 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 || null == head.next) {
            return head;
        }
        
        ListNode ptr = head;
        while (ptr.next != null) {
            if (ptr.val == ptr.next.val) {
                ptr.next = ptr.next.next;
            } else {
                ptr = ptr.next;
            }
        }
        
        return head;
    }
}