题解 | #删除有序链表中重复的元素-II#

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @return ListNode类
     */
    ListNode* deleteDuplicates(ListNode* head) {
        // write code here
        if(head == nullptr || head->next == nullptr) {
            return head;
        }
        ListNode *theHead = new ListNode(-1001);
        theHead->next = head;
        ListNode *pre = theHead;
        ListNode *rear = pre->next->next;
        while(true) {
            bool flag = false;
            while(rear->val == pre->next->val) {
                flag = true;
                rear = rear->next;
                if(rear == nullptr) {
                    if(pre->next->val == pre->next->next->val) {
                        pre->next = nullptr;
                    }
                    return theHead->next;
                }
            }
            if(flag) {
                pre->next = rear;
            }
            if(pre->next == nullptr) {
                return theHead->next;
            }
            else {
                if(pre->next->next == nullptr) {
                    return theHead->next;
                }
                else{
                    if(pre->next->val == pre->next->next->val) {
                        continue;
                    }
                    else {
                        pre = pre->next;
                    }
                }
            }
            rear = pre->next->next;
            if(rear == nullptr) {
                return theHead->next;
            }
        }
    }
};