思路：Remove Duplicates from Sorted List II

II比I难在需要删除所有的重复节点。这里需要注意两点：

1. 头节点也可能被删除。可以使用dummy节点来简化。

2. 如果采用与I类似的思路来删除当前节点后所有的重复节点，则完成后还需要把当前节点也删除。因此需要有一个变量来记录当前节点是否有重复。

class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        ListNode *dummy = new ListNode(0);
        dummy->next = head;
        ListNode *pre = dummy, *cur = head;
        bool duplicate = false;

        while(cur) {
            if(cur->next && cur->val==cur->next->val) {
                ListNode *temp = cur->next;
                cur->next = temp->next;
                delete temp;
                duplicate = true;
            }
            else if(duplicate) {
                pre->next = cur->next;
                delete cur;
                cur = pre->next;
                duplicate = false;
            }
            else {
                pre = cur;
                cur = cur->next;
            }
        }

        head = dummy->next;
        delete dummy;
        return head;
    }
};