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;        
    }
};