LeetCode_ 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 *merge(ListNode *head1, ListNode * head2){

        if(head1 == NULL) return head2;
        if(head2 == NULL) return head1;
        ListNode *head = new ListNode() ;
        ListNode *p, *p1, *p2;
        p = head, p1 = head1, p2 = head2;
        while(p1 != NULL && p2 != NULL){
            if(p1->val < p2->val){
                p->next = p1;
                p = p1;
                p1 = p1->next;
            }else{
                p->next = p2;
                p = p2;
                p2 = p2->next;
            }
        }
        p->next = p1 == NULL ? p2 : p1;
        p = head->next;

        delete head;
        return p;
    }
    ListNode *mergeKLists(vector<ListNode *> &lists) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int n = lists.size();
        if(n < ) return NULL;
        int len = ;
        while(len < n){
            for(int i = ; i + len < n; i += len * ){
                    lists[i] = merge(lists[i], lists[i+len]);
            }
            len = len<<;
        }
        return lists[];
    }
};

归并的思路来做。