[LeetCode] 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.

关键是记录三个指针，当前指针p, p的父亲pPre, pPre的父亲pPrePre。

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (head == NULL)
            return NULL;
        
        ListNode *pPrePre = NULL;    
        ListNode *pPre = NULL;
        ListNode *p = head;
        int key = INT_MAX;
        
        while(p)
        {
            if (key != p->val)
            {
                key = p->val;
                pPrePre = pPre;
                pPre = p;
                p = p->next;
            }
            else
            {
                if (pPre && pPre->val == key)
                {
                    ListNode *pNext = p->next;
                    if (pPrePre)
                        pPrePre->next = pNext;
                    if (head == pPre)
                        head = pNext;                   
                    delete pPre;
                    delete p;
                    pPre = pPrePre;
                    p = pNext;                       
                }
                else
                {
                    ListNode *pNext = p->next;
                    if (pPre)
                        pPre->next = pNext;
                    if (head == p)
                        head = pNext;
                    delete p;
                    p = pNext;
                }
            }
        }
        
        return head;
    }
};