[LeetCode] 23. Merge k Sorted Lists

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

/**
 * 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) {
        int k = lists.size();
        if (k == 0) return NULL;
        
        while (k > 1){
            for (int i = 0; i < k /2; i++){
                lists[i] = mergeTwoLists(lists[i], lists[k-1-i]);
            }
            if (k%2){
                k = k/2 + 1;
            }else{
                k = k / 2;
            }
        }
        
        return lists[0];
    }
    
private:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *dummy = new ListNode(0);
        ListNode *ret = NULL;
        
        
        ListNode *p1 = l1;
        ListNode *p2 = l2;
        
        ListNode *p = dummy;
        
        while(p1 && p2){
            if (p1->val < p2->val){
                p->next = p1;
                p1 = p1->next;
            }else{
                p->next = p2;
                p2 = p2->next;
            }
            p = p->next;
        }
        
        p1 = (!p1) ? p2 : p1;
        
        while (p1){
            p->next = p1;
            // don't forget to move p1 !!
            p1 = p1->next;
            p = p->next;
        }
        
        ret = dummy->next;
        delete dummy;
        return ret;
    }
};