双指针:空间复杂度O(1),时间复杂度O(n)
public ListNode deleteDuplicates (ListNode head) { if(head==null){ return null; } // write code here ListNode pre = head; ListNode preNext = head.next; while(preNext!=null){ if(pre.val!=preNext.val){ pre.next=preNext; pre = preNext; } preNext=preNext.next; } pre.next=preNext; return head; }