链表 Remove Duplicates from Sorted List II

思想：

注意使用头结点来简化编程；

删除的节点要delete，避免内存泄露；

/** * 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 || head->next == NULL) return head;        ListNode dummy(0);        dummy.next = head;        ListNode *prev = &dummy;        ListNode *cur = head;        while(cur != NULL) {            bool dup = false;            while(cur->next != NULL && cur->val == cur->next->val) {                dup =true;                ListNode *tmp = cur;                cur = cur->next;                delete tmp;//necessary, else memory leak            }            if(dup) {                ListNode *tmp = cur;                cur = cur->next;                delete tmp;                continue;            }            prev->next = cur;            prev = prev->next;            cur = cur->next;        }        prev->next = cur;        return dummy.next;            }};