/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 * 
 * Note: The returned array must be malloced, assume caller calls free().
 */

#define N 10005 
typedef struct ListNode* Node;

int st[N] = {0};

struct ListNode* filterList(struct ListNode* head) {
    st[abs(head->val)] = 1;
    Node p = head;
    while(p->next) {
        if(st[abs(p->next->val)]) {
            Node q = p->next;
            p->next = q->next;
            free(q);
        } else {
            p = p->next;
            st[abs(p->val)] = 1;
        }
    }
    return head;
}