solution.h

#include <vector>
using std::vector;
#include <cstddef>

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode *head = NULL, **lastPtrRef = &head;
        for (;l1 && l2; lastPtrRef = &((*lastPtrRef)->next)) {
            if (l1->val <= l2->val) { *lastPtrRef = l1; l1 = l1->next; }
            else { *lastPtrRef = l2; l2 = l2->next; }
        }
        *lastPtrRef = l1 ? l1 : l2;
        return head;
    }

    using vecNodeCIter = vector<ListNode *>::const_iterator;
    ListNode *mergeKLists(vecNodeCIter beg, vecNodeCIter end) {
        if (beg == end) return NULL;
        else if (std::distance(beg, end) == 1) return *beg;
        else if (std::distance(beg, end) == 2) return mergeTwoLists(*beg, *std::next(beg));
        else return mergeTwoLists(mergeKLists(beg, beg + std::distance(beg, end)/2), mergeKLists(beg + std::distance(beg, end)/2, end));
    }
public:
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        return mergeKLists(lists.cbegin(), lists.cend());
    }
};