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.

没什么太多讲的，可以使用递归和迭代两种方法来做，要仔细考虑各种输入情况。code如下：

class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        if(head == NULL)
            return NULL;
        ListNode *first = head,*second = NULL,*result = NULL;
        bool isDup = false;
        while(first!=NULL)
        {
            isDup = false;
            while(first->next != NULL && first->val == first->next->val)
            {
                isDup = true;
                first = first->next;
            }
            if(!isDup)
            {
                if(second == NULL)
                {
                    second = first;
                    if(result == NULL)
                        result = second;
                }
                else
                {
                    second->next = first;
                    second = second->next;
                }
            }
            first = first->next;
        }
        if(second!=NULL)
            second->next = NULL;
        return result;
    }
};