leetCode练习（82）

 题目：Remove Duplicates from Sorted List II

难度：medium

问题描述：

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

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

解题思路：将重复的节点全部去除。解题技巧是设置一个头结点extra，使得extra.next=head。这样可以在head也被删时灵活表示。最后输出extra.next即可。

具体代码如下：

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode deleteDuplicates(ListNode head) {
        if(head==null||head.next==null){
    	   return head;
       }
       boolean gate=false;
       ListNode extra=new ListNode(0);
       extra.next=head;
       ListNode last=extra,index=head;
       while(true){
    	   if(index.next.val==index.val){
    		   index=index.next;
    		   gate=true;
    	   }else{	   
    		   index=index.next;
    		   if(gate){
    			   last.next=index;
    			   gate=false; 
    		   }else{
    			   last=last.next;
    		   }
    		   
    	   }
    	   if(index.next==null){
    		   if(gate){
    			   last.next=null;
    		   }
    		   break;
    	   }
       }
       return extra.next;
    }
}