K-way Merge

mergeKLists.cpp

// min-heap
ListNode* mergeKLists(vector<ListNode*>& lists) {
    auto comp = [](ListNode* a, ListNode* b) { return a->val > b->val; };
    priority_queue<ListNode*, vector<ListNode*>, decltype(comp)> pq(comp);
    ListNode dummy(0);
    ListNode *cur = &dummy;
    for(ListNode *list : lists){
        if(list) pq.push(list);
    }
    while(!pq.empty()){
        cur->next = pq.top();
        pq.pop();
        cur = cur->next;
        if(cur->next) //cur won't be nulltpr as cur is the element poped from the queue which is always non-nullptr, so we only need to check if its next is nullptr
            pq.push(cur->next);
    }
    return dummy.next;
}

// devide and conquer
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2){
    ListNode dummy(0);
    ListNode *cur = &dummy;
    while(l1 && l2){
        if(l1->val < l2->val){
            cur->next = l1;
            l1 = l1->next;
        }else{
            cur->next = l2;
            l2 = l2->next;
        }
        cur = cur->next;
    }
    if(l1) cur->next = l1;
    if(l2) cur->next = l2;
    return dummy.next;
}

ListNode* mergeKLists(vector<ListNode*>& lists) {
    int n = lists.size();
    while(n > 1){
        int k = (n + 1) / 2;
        for(int i = 0; i < n / 2; i++){
            lists[i] = mergeTwoLists(lists[i], lists[i+k]);
        }
        n = k;
    }
    return lists.empty() ? nullptr : lists[0];
}