Merge Two Sorted Lists

初阶。

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.

进阶。

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

简洁的方式，使用递归：

public:
    ListNode* Merge(ListNode* pHead1, ListNode* pHead2)
    {
        if (pHead1 == NULL)
        	return pHead2;
        if (pHead2 == NULL)
            <span style="white-space:pre">	</span>return pHead1;
        if (pHead1->val < pHead2->val)
        {
            pHead1->next = Merge(pHead1->next,pHead2);
            return pHead1;
        }else
        {
            pHead2->next = Merge(pHead1,pHead2->next);
            return pHead2;
        }
        
    }
};

#include <iostream>
#include <vector>
using namespace std;

struct LinkNode
{
	int data;
	LinkNode *next;
	LinkNode(int x):data(x),next(nullptr){}
};

LinkNode* mergeTwoList(LinkNode *l1,LinkNode *l2)
{
	LinkNode *p1 = l1,*p2 = l2,*head,*pre;
	LinkNode *dummy = new LinkNode(-1);
	pre = dummy;
	
	while(p1&&p2)
	{
		if(p1->data < p2->data)
		{
			pre->next = p1;
			p1 = p1->next;			
		}
		else
		{
			pre->next = p2;
			p2 = p2->next;
		}
		pre = pre->next;
	}

	if(p1)
	        pre->next = p1;

	if(p2)
	        pre->next = p2;

	head = dummy->next;
	delete dummy;
	return head;
}

//merge k个list
LinkNode* mergeKList(vector<LinkNode*> &lists)
{
	if(lists.size() == 0) return NULL;

	LinkNode *p = lists[0];
	for(unsigned int i=1;i<lists.size();i++)
	{
		p = mergeTwoList(p,lists[i]);
	}
	return p;
}

//对链表初始化
LinkNode *initLink(LinkNode *dummy,int array[],int len)
{
	LinkNode *head,*tmp;//dummy表示哑节点，head一般表示头指针，它指向第一个节点。
	tmp = dummy;
	
	//用尾插法
	for(int i = 0;i < len;i++)
	{
		LinkNode *p = new LinkNode(array[i]);
		tmp->next = p;
		tmp = tmp->next;
	}
	
	head = dummy->next;
	delete dummy;
	return head;
}

//显示list
void showList(LinkNode *head)
{
	while(head)
	{
		printf("%d ",head->data);
		head = head->next;
	}
	printf("\n");
}

//摧毁链表
void destroyList(LinkNode *head)
{
	LinkNode *dummy = new LinkNode(-1);
	dummy->next = head;
	LinkNode *pre,*cur,*tmp;
	pre = dummy;
	cur = pre->next;
	while(cur)
	{
		tmp = cur->next;
		pre->next = tmp;
		delete cur;
		cur = tmp;
	}
	delete dummy;
	printf("The list has been destroyed!\n");
}

void main()
{
	LinkNode *dummy1 = new LinkNode(-1);
	int array1[] = {1,3,5,7,9};
	int len1 = sizeof(array1)/sizeof(int);
	LinkNode *head1 = initLink(dummy1,array1,len1);
	showList(head1);

	LinkNode *dummy2 = new LinkNode(-2);
	int array2[] = {2,4,6,8,10,11,12};
	int len2 = sizeof(array2)/sizeof(int);
	LinkNode *head2 = initLink(dummy2,array2,len2);
	showList(head2);

	LinkNode *dummy3 = new LinkNode(-3);
	int array3[] = {2,7,10,13,15,20,26,33,100,120};
	int len3 = sizeof(array2)/sizeof(int);
	LinkNode *head3 = initLink(dummy3,array3,len3);
	showList(head3);

	//LinkNode *head = mergeTwoList(head1,head2);
	//showList(head);

	vector<LinkNode*> lists;
	lists.push_back(head1);
	lists.push_back(head2);
	lists.push_back(head3);

	LinkNode *head = mergeKList(lists);
	showList(head);

	destroyList(head);
}