83. Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.

Given 1->1->2->3->3, return 1->2->3.

需要注意只存在一个节点的情况，

/**
 * 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 || head->next == NULL) {
            return head;
        }
        ListNode* dummy = new ListNode(0);
        ListNode* node = dummy;
        while (head != NULL && head->next != NULL) {
            while (head != NULL && head->next != NULL &&
                   head->val == head->next->val) {
                head = head->next;
            }
            node->next = head;
            node = node->next;
            head = head->next;
        }
        return dummy->next;
    }
};