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// two lists are sorted
vector<int> find_common_element_of_two_sorted_lists(ListNode *l1, ListNode *l2) {
    vector<int> res;
    if(!l1 || !l2) return res;
    ListNode *p1 = l1, *p2 = l2;
    unordered_set<int> se;
    while(p1 && p2) {
        if(p1->val == p2->val) {
            se.insert(p1->val);
            p1 = p1->next;          // forgot!!!
            p2 = p2->next;          // forgot!!!
        }
        else if(p1->val < p2->val)
            p1 = p1->next;
        else
            p2 = p2->next;
    }
    for(auto i : se) res.push_back(i);
    return res;
}

// two lists are unsorted
vector<int> find_common_element_of_two_unsorted_lists(ListNode *l1, ListNode *l2) {
    vector<int> res;
    if(!l1 || !l2) return res;
    unordered_set<int> se;
    ListNode *p1 = l1, *p2 = l2;
    for(; p1; p1=p1->next) 
        se.insert(p1->val);
    for(; p2; p2=p2->next) {
        if(se.find(p2->val) != se.end())
            res.push_back(p2->val);
    }
    return res;
}