代码实现：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
struct ListNode* deleteDuplicates(struct ListNode *head) {
    if (head == NULL || head->next == NULL) {
        return head;
    }
    struct ListNode *h = malloc(sizeof(*h)); // 创建虚拟头结点
    h->next = head;
    struct ListNode *pre = h;
    struct ListNode *p = head, *pp = head->next, *pc = NULL;
    while (pp) {
        if (p->val != pp->val) {
            pp = pp->next;
            pre = p;
            p = p->next;
        } else {
            while (pp && p->val == pp->val) {
                pp = pp->next;
            }
            pre->next = pp;
            while (p != pp) {
                pc = p;
                p = p->next;
                pc->next = NULL;
                free(pc);
            }
            if (pp) {
                pp = pp->next;
            }
        }
    }
    return h->next;
}