给定一个排序链表，删除所有重复的元素，使得每个元素只出现一次。

/**
 * 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(head == null) return null;

        ListNode p = head;

        ListNode currentNode = head.next;

        ListNode preNode = null;


        while(p!=null){
            preNode = p;
            currentNode = p.next;
            while(currentNode!=null){
                if(currentNode.val!=p.val){
                    currentNode = currentNode.next;
                    preNode = preNode.next;
                }else{
                    preNode.next = currentNode.next;
                    currentNode = currentNode.next;
                }
            }
            p = p.next;
            
        }

        return head;
    }
}

注意到是已经拍过序的链表了所有第二种方案更好

/**
 * 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(head == null) return null;

        ListNode currentNode = head;
        while(currentNode != null && currentNode.next!=null){
            if(currentNode.val == currentNode.next.val){
                currentNode.next = currentNode.next.next;
            }else{
                currentNode = currentNode.next;
            }
        }

        return head;
    }
}