23. Merge k Sorted Lists

Posted 2020-09-07 为了更优秀的你，加油！

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* mergeTwoLists(ListNode* l1,ListNode* l2){
        if(!l1)return l2;
        if(!l2)return l1;
        ListNode *p=l1,*q=l2,*head=NULL,*tail=NULL;
        while(p&&q){
            if(p->val<q->val){
                if(!head){head=p;}
                else tail->next=p;
                tail=p;
                p=p->next;
            }
            else {
                if(!head){head=q;}
                else tail->next=q;
                tail=q;
                q=q->next;
            }
        }
        if(p)tail->next=p;
        else tail->next=q;
        return head;
    }
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if(lists.size()==0)return NULL;
        if(lists.size()==1)return lists[0];
        if(lists.size()==2)return mergeTwoLists(lists[0],lists[1]);
        vector<ListNode*>subList(lists.begin(),lists.begin()+lists.size()/2);
        vector<ListNode*>rest(lists.begin()+lists.size()/2,lists.begin()+lists.size());
        return mergeTwoLists(mergeKLists(subList),mergeKLists(rest));
    }
};