【Merge K Sorted Lists】cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if (lists.size()==0) return NULL;
        return Solution::mergeK(lists, 0, lists.size()-1);
    }
    static ListNode* mergeK(vector<ListNode*>& lists, int begin, int end )
    {
            if ( begin==end ) return lists[begin];
            if ( (begin+1)==end ) return Solution::mergeTwo(lists[begin], lists[end]);
            int mid = ( begin + end ) / 2;
            ListNode *firstHalf = Solution::mergeK(lists, begin, mid);
            ListNode *secondHalf = Solution::mergeK(lists, mid+1, end);
            return Solution::mergeTwo(firstHalf, secondHalf);
    }
    static ListNode* mergeTwo( ListNode *h1, ListNode *h2 )
    {
            ListNode dummy(-1);
            ListNode *p = &dummy;
            while ( h1 && h2 ){
                if ( h1->val<h2->val ){
                    p->next = h1;
                    h1 = h1->next;
                }
                else{
                    p->next = h2;
                    h2 = h2->next;
                }
                p = p->next;
            }
            p->next = h1 ? h1 : h2;
            return dummy.next;
    }
};