给定一个已排序的链表的头 head ， 删除所有重复的元素，使每个元素只出现一次 。返回 已排序的链表 。

示例 1：

输入：head = [1,1,2]
输出：[1,2]

示例 2：

输入：head = [1,1,2,3,3]
输出：[1,2,3]

提示：

• 链表中节点数目在范围 [0, 300] 内
• 100 <= Node.val <= 100
• 题目数据保证链表已经按升序 排列

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        ListNode slow = head, fast = head;
        while (fast != null ){
            if(slow.val != fast.val){
                slow.next = fast;
                slow = slow.next;
            }
            if (slow.val == fast.val && fast.next == null){
                slow.next = null;
            }
            fast = fast.next;
        }
        return head;
    }
}