[LeetCode]Merge k Sorted Lists

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

思考：合并双链表的基础上增加循环。

/**

 * 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 *p=head1;

		ListNode *q=head2;

		ListNode *newhead=NULL;	

		ListNode *r=newhead;

		while(p&&q)

		{

			ListNode *newnode=new ListNode(NULL);

			if(p->val<q->val)

			{

				newnode=p;

				p=p->next;

			}

			else

			{

				newnode=q;

				q=q->next;

			}

			if(newhead==NULL)

			{

				newhead=newnode;

				r=newnode;

			}

			else

			{

				r->next=newnode;

				r=r->next;

			}

		}

		if(p) r->next=p;

		else  r->next=q;

		return newhead;		

	}

    ListNode *mergeKLists(vector<ListNode *> &lists) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        ListNode *newhead=NULL;

		for(int i=0;i<lists.size();i++)

		{

			newhead=merge(newhead,lists[i]);

		}

		return newhead;

    }

};