Remove Duplicates from Sorted List II

Remove Duplicates from Sorted List II

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.

解析：

从前向后遍历，使用三个指针进行操作，画图理解

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;        if (head->next==NULL) return head;        ListNode* first,*second;        ListNode* prehead=new ListNode(0);        prehead->next=head;        first=head;        second=first->next;        int flag=0;        ListNode* prefirst=prehead;        while(second)        {           if (first->val==second->val)           {               flag=1;               prefirst->next=second;               first=second;               second=second->next;           }           else           {               if (flag)               {                   prefirst->next=second;                   first=second;                   second=second->next;               }               else               {                   prefirst=first;                   first=second;                   second=second->next;               }               flag=0;           }        }        if (flag)        {            prefirst->next=NULL;        }        return prehead->next;    }};