Leetcode 23. Merge k Sorted Lists

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */ 用排序的方法来解决问题。
class Solution {
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        std::vector<ListNode*> node_vec;
        ListNode* head = NULL;
        for(int i=0 ;i<lists.size(); ++i)
        {
            head = lists[i];
            while(head)
            {
                node_vec.push_back(head);
                head = head->next;
            }
        }
        if(node_vec.size()==0)
            return NULL;
        std::sort(node_vec.begin(), node_vec.end(), [](const ListNode*a, const ListNode* b){return a->val < b->val;});
        
        int i=0;
        for(;i<node_vec.size(); ++i)
        {
            node_vec[i]->next = node_vec[i+1];
        }
        node_vec[node_vec.size()-1]->next = NULL;
        
        return node_vec[0];
    }  
};

/**
 * 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) {
        ListNode temp_head(0);
        ListNode * pre = &temp_head;
        while(l1 && l2)
        {
            if(l1->val < l2->val)
            {
                pre->next = l1;
                l1 = l1->next;
            }
            else
            {
                pre->next = l2;
                l2 = l2->next;
            }
            pre = pre->next;
        }
        if(l1)
        {
            pre->next = l1;
        }
        if(l2)
        {
            pre->next = l2;
        }
        return temp_head.next;
    }
    
    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]);
        
        int mid = (lists.size()/2);
        
        vector<ListNode*> sub1_lists;
        vector<ListNode*> sub2_lists;
        
        for(int i=0; i<mid; ++i)
            sub1_lists.push_back(lists[i]);
        for(int i=mid; i<lists.size(); ++i)
            sub2_lists.push_back(lists[i]);
        
        ListNode * l1 = mergeKLists(sub1_lists);
        ListNode * l2 = mergeKLists(sub2_lists);
        
        return mergeTwoLists(l1, l2);
    }  
};