82. 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

.

【思路】创建三个指针，当前节点p,当前节点的父节点pPre, 当前节点的父节点的父节点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) {
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;
}
};