合并k个排序链表

样例

给出3个排序链表[2->4->null,null,-1->null]，返回 -1->2->4->null

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/** * Definition of ListNode * class ListNode { * public: *     int val; *     ListNode *next; *     ListNode(int val) { *         this->val = val; *         this->next = NULL; *     } * } */class Solution {public:    /**     * @param lists: a list of ListNode     * @return: The head of one sorted list.     */    ListNode *merge2Lists(ListNode* a,ListNode* b){        if(a==NULL) return b;        if(b==NULL) return a;        ListNode *dummy=new ListNode(-1);        ListNode *tmp=dummy;        while(a&&b){            if(a->val<=b->val){                tmp->next=a;                a=a->next;            }else{                tmp->next=b;                b=b->next;            }            tmp=tmp->next;        }        if(a) tmp->next=a;        if(b) tmp->next=b;        return dummy->next;            }    ListNode *mergeKLists(vector<ListNode *> &lists) {        // write your code here        int n=lists.size();        ListNode* res=NULL;        if(n==0) return NULL;        if(n==1) return lists[0];        for(int i=0;i<n;i++){            res=merge2Lists(res,lists[i]);        }        return res;    }};