/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 * 
 * Note: The returned array must be malloced, assume caller calls free().
 */
int abs(int a){
    return a > 0 ? a : -a; 
}
struct ListNode* filterList(struct ListNode* head) {
    struct ListNode *p, *q;
    int st[10010] = {0};
    st[abs(head -> val)] = 1;
    for (p = head; p -> next;){
        int x = abs(p -> next -> val);
        if (st[x]){
            q = p -> next;
            p -> next = q -> next;
            free(q);
        }
        else{
            p = p -> next;
            st[x] = 1;
        }
    }
    return head;
}