[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.

用一个值val来标记当前要删除的元素，如果结点的值等于val，就删除掉。如果当前值与下一个结点的值相等，就将val设成当前值。

/**
 * Definition of ListNode
 * class ListNode {
 * public:
 *     int val;
 *     ListNode *next;
 *     ListNode(int val) {
 *         this->val = val;
 *         this->next = NULL;
 *     }
 * }
 */
class Solution{
public:
    /**
     * @param head: The first node of linked list.
     * @return: head node
     */
    ListNode * deleteDuplicates(ListNode *head) {
        // write your code here
        if (head == NULL) return NULL;
        ListNode dummy(0);
        ListNode *cur = head, *pre = &dummy, *tmp;
        int val = head->val - 1;
        while (cur != NULL) {
            if (cur->val == val || (cur->next != NULL && cur->val == cur->next->val)) {
                val = cur->val;
                tmp = cur->next;
                delete cur;
                cur = tmp;
            } else {
                pre->next = cur;
                cur = cur->next;
                pre = pre->next;
            }
        }
        pre->next = NULL;
        return dummy.next;
    }
};