【Leetcode長征系列】Remove Duplicates from Sorted List II

原題：

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving onlydistinct 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.

代碼：

/**
 * 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;
        if(head->next==NULL) return head;
        ListNode *tmp = head, *pre;
        
        pre = NULL;
        while(tmp){
            if (tmp->val==tmp->next->val) {
                ListNode *s = tmp->next;
                while (s->next!=NULL && s->val==s->next->val){
                    s = s->next;
                }
                if(pre==NULL){
                    if(s->next!=NULL){
                        head = s->next;
                        tmp = head;
                    }
                    else return NULL;
                }
                else{
                    pre->next = s->next;
                    tmp = pre->next;
                }
            }
            else {
                pre = tmp;
                tmp = tmp->next;
            }
                
            if (tmp==NULL || tmp->next==NULL) break;
        }
        return head;
    }
};

需要注意的特殊情況：

1. {1}

2. {1->2}

3. {1->1->2}

要注意pre的初始值！NULL最佳，本次情況下不適合用head作爲初始值。