LeetCode(82)题解: Remove Duplicates from Sorted List II

https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/

题目：

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.

思路：

需要两个指针，a用来纪录新链表，b用来遍历老链表，因为只有删除操作，需要一个flag标记ab段要不要删除。此外要注意头节点的处理。

AC代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        if(head==NULL)
            return NULL;
        ListNode* newhead=new ListNode(0);  // to handle head
        newhead->next=head;
        ListNode* a=newhead,*b=newhead->next;
        int now=head->val;
        bool occur_flag=false;
        while(b->next!=NULL){
            if(b->next->val==now){
                occur_flag=true;
            }
            else{
                if(occur_flag==true){
                    a->next=b->next;  //delete duplicate numbers
                    b=a;    //update pointers
                }
                else{
                    a=b;
                }
                occur_flag=false;
                now=b->next->val;
            }
            b=b->next;
        }
        if(occur_flag==true){
            a->next=b->next;  //delete duplicate numbers
            b=a;    //update pointers
        }
        else{
            a=b;
        }
        return newhead->next;
    }
};