/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 *	ListNode(int x) : val(x), next(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
     *
     * 
     * @param head ListNode类 
     * @return ListNode类
     */
    ListNode* deleteDuplicates(ListNode* head) {
        if (head == nullptr) return nullptr;
        ListNode* left = head, *right = head;
        while (right!=nullptr) {
            if (left == right or left->val == right->val) {
                right = right->next;
                continue;
            }
            left->next = right;
            left = right;
        }
        left->next = right;
        left = right;
        return head;
    }
};