21 Merge Two Sorted Lists &&23. Merge k Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

struct ListNode {
      int val;
      ListNode *next;
      ListNode(int x) : val(x), next(NULL) {}
  };
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
       if(!l1)
            return l2;
        if(!l2)
            return l1;
        ListNode *newHead=new ListNode(0);
          ListNode *p=newHead;
        while(l1&&l2)
        {
            if(l1->val<=l2->val)
        {
             ListNode *newNode=new ListNode(l1->val);
                p->next=newNode;
                p=newNode;
            l1=l1->next;
            
            
        }
           
        else
        {
             ListNode *newNode=new ListNode(l2->val);
                p->next=newNode;
                p=newNode;
            l2=l2->next;
            
        }
        }
            
        while(l1)
        {
           ListNode *newNode=new ListNode(l1->val);
                p->next=newNode;
                p=newNode;
            l1=l1->next;
            
        }
        while(l2)
        {
            ListNode *newNode=new ListNode(l2->val);
                p->next=newNode;
                p=newNode;
            l2=l2->next;
        }
        
       return newHead->next;
       
       
        
    }

把这个问题复杂化一点：

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

在两个链表合并的基础上，运用归并排序的思想

struct ListNode {
      int val;
      ListNode *next;
      ListNode(int x) : val(x), next(NULL) {}
  };

//这部分不变，两个链表的合并
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
       if(!l1)
            return l2;
        if(!l2)
            return l1;
        ListNode *newHead=new ListNode(0);
          ListNode *p=newHead;
        while(l1&&l2)
        {
            if(l1->val<=l2->val)
        {
             ListNode *newNode=new ListNode(l1->val);
                p->next=newNode;
                p=newNode;
            l1=l1->next;
            
            
        }
           
        else
        {
             ListNode *newNode=new ListNode(l2->val);
                p->next=newNode;
                p=newNode;
            l2=l2->next;
            
        }
        }
            
        while(l1)
        {
           ListNode *newNode=new ListNode(l1->val);
                p->next=newNode;
                p=newNode;
            l1=l1->next;
            
        }
        while(l2)
        {
            ListNode *newNode=new ListNode(l2->val);
                p->next=newNode;
                p=newNode;
            l2=l2->next;
        }
        
       return newHead->next;
       
       
        
    }
 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]=mergeTwoLists(lists[i],lists[i+k]);
		 }
		  n=k;
	 }
	 return lists[0];


 }

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