Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

class Solution{    public:    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {    if(l1 == nullptr){        return l2;    }    if(l2 == nullptr){        return l1;    }    if(l1->val <= l2->val){        l1->next = mergeTwoLists(l1->next, l2);        return l1;    }    else{        l2->next = mergeTwoLists(l1, l2->next);        return l2;    }}ListNode *mergeKLists(vector<ListNode *> &lists) {    if(lists.empty()){        return nullptr;    }    while(lists.size() > 1){        lists.push_back(mergeTwoLists(lists[0], lists[1]));        lists.erase(lists.begin());        lists.erase(lists.begin());    }    return lists.front();}};

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