Merge k Sorted Lists 合并多个链表 归并 vector/copy

Merge k Sorted Lists

Merge k

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
//归并排序 O(NlogK)
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        if(lists.size()==0||lists.empty())
            return NULL;
        if(lists.size()==1)
            return lists[0];
            
        int len=lists.size();
        int mid=(len-1)/2;
        
        vector<ListNode *> L,R;
        L.resize(mid+1);
        R.resize(len-mid-1);
        copy(lists.begin(),lists.begin()+mid+1,L.begin());
        copy(lists.begin()+mid+1,lists.end(),R.begin());
        ListNode *left=mergeKLists(L);
        ListNode *right=mergeKLists(R);
        
        return merge(left,right);
    }
    
    
    ListNode *merge(ListNode *a,ListNode *b)
    {
        if(a==NULL&&b==NULL)
              return NULL;
        if(a==NULL)
              return b;
        if(b==NULL)
              return a;
              
        ListNode *head=new ListNode(0);
        ListNode *ans=head;
        
        while(a && b)
        {
            if(a->val < b->val)
            {
                ans->next=a;
                a=a->next;
            }
            else
            {
                ans->next=b;
                b=b->next;
            }
            ans=ans->next;
        }
        
        while(a)
        {
            ans->next=a;
            a=a->next; 
            ans=ans->next;
        }
        
         while(b)
        {
            ans->next=b;
            b=b->next; 
            ans=ans->next;
        }
        
        return head->next;
    }
};