[Leetcode] Merge k Sorted Lists

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; */class Solution {public:    ListNode* merge2lists(ListNode* l1, ListNode* l2){        ListNode* Newhead=new ListNode(0);        ListNode* pre=Newhead;                while(l1!=NULL||l2!=NULL){            int va=l1==NULL? INT_MAX:l1->val;            int vb=l2==NULL? INT_MAX:l2->val;                        if(va>=vb){                pre->next=l2;                l2=l2->next;            }            else{                pre->next=l1;                l1=l1->next;            }            pre=pre->next;        }        pre=Newhead->next;        delete Newhead;        return pre;    }  /*      ListNode *mergeKLists(vector<ListNode *> &lists) {        if(lists.empty()) return NULL;        //ListNode dummy(0);        ListNode* pre=lists[0];        //dummy.next=pre;                for(int i=1; i<lists.size(); i++){            pre=merge2lists(pre,lists[i]);        }                return pre;    }    */                ListNode *mergeKLists(vector<ListNode *> &lists) {                int n=lists.size();        if(n==0)return NULL;        while(n>1){            int k=(n+1)/2;            for (int i=0;i<n/2;i++){                lists[i]=merge2lists(lists[i],lists[i+k]);            }            n=k;        }        return lists[0];    }};