主要问题就在Merge函数里，如何修改

#include<iostream>
using namespace std;

template<class T>
class LinkNode
{
	template<class T>
	friend class LinkList;
public:
	LinkNode()
	{
		next =NULL;
	}
	T data;
	LinkNode<T>*next;
};

template<class T>
class LinkList
{
public:
	LinkList();
	~LinkList();
	LinkList<T>&Insert(int k,const T&x);
	LinkList<T>&DeleteByIndex(int k, T&x);
	int GetLength()const;
	bool GetData(int k,T &x);
	void Merge(LinkList<T> L1,LinkList<T> L2,LinkList<T>&L3);
	void OutPut(ostream& out);
	LinkNode<T>*head;
};

template<class T>
LinkList<T>::LinkList()
{
	head=new LinkNode<T>();
}

template<class T>
LinkList<T>::~LinkList()
{
	T x;
	int len=GetLength();
	for(int i=len;i>=1;i--)
		DeleteByIndex(i,x);
	delete head;
}

template <class T>
int LinkList<T>::GetLength()const
{
	int length=0;
	LinkNode<T>*p=head->next;
	while(p)
	{
		length++;
		p=p->next;
	}
	return length;
}

template<class T>
LinkList<T>&LinkList<T>::DeleteByIndex(int k,T &x)
{
	if(GetData(k,x))
	{
		LinkNode<T> *p=head;
		LinkNode<T> *q=NULL;
		for(int i=1;i<k;i++)
			p=p->next;
		q=p->next;
		p->next=q->next;
		delete q;
	}
	else
		cout<<"元素下标越界，删除失败"<<endl;
	return *this;
}

template<class T>
bool LinkList<T>::GetData(int k,T&x)
{
	LinkNode<T>*p=head->next;
	int index=1;
	if(k<1||k>GetLength())
		return false;
	while(p!=NULL&&index<k)
	{
		index++;
		p=p->next;
	}
	if(p==NULL)
		return false;
	else
	{
		x=p->data;
		return true;
	}
}



template<class T>
void LinkList<T>::OutPut(ostream &out)
{
	LinkNode<T>*p=head->next;
	while(p!=NULL)
	{
		out<<p->data<<endl;
		p=p->next;
	}
}

template<class T>
ostream &operator<<(ostream &out ,LinkList<T>&x)
{
	x.OutPut(out);
	return out;
}

template<class T>
LinkList<T>&LinkList<T>::Insert(int k,const T&x)
{
	LinkNode<T>*p=head;
	LinkNode<T>*newNode=new LinkNode<T>;
	newNode->data=x;
	int len=GetLength();
	if(k<1||k>len+1)
		cout<<"元素下标越界，添加元素失败";
	else
	{
		for(int i=1;i<k;i++)
			p=p->next;
		newNode->next=p->next;
		p->next=newNode;
	}
	return *this;
}

template<class T>
void Merge(LinkList<T> L1,LinkList<T> L2,LinkList<T>&L3)
{
	LinkNode<T>*p=L3.head;
	LinkNode<T>*newNode=new LinkNode<T>;   
	LinkNode<T>*m=L1.head;
	LinkNode<T>*n=L2.head;
	for(int i=0;i<L1.GetLength()+L2.GetLength();i++)
	{
		
		while(m!=NULL&&m->data<n->data)
		{
			p->data=m->data;
			m=m->next;
			p=p->next;
		}
		while(n!=NULL&&n->data<m->data)
		{
			p->data=n->data;
			n=n->next;
			p=p->next;
		}
		while(m=NULL)
		{
			p->data=n->data;
		}
		while(n=NULL)
		{
			p->data=m->data;

		}
	}

}

#include"Linklist.h"
#include<iostream>
using namespace std;


int main()
{
	
	LinkList<int>L1,L2,L3;
	L1.Insert(1,100);
	L1.Insert(2,200);
	L1.Insert(3,300);
	L1.Insert(4,400);
	L2.Insert(1,50);
	L2.Insert(2,150);
	L2.Insert(3,250);
	L2.Insert(4,270);

    Merge(L1,L2,L3);
	cout<<"当前链表1和2的元素为:"<<endl<<L1<<L2<<endl;
	cout<<"元素合并之后的链表是："<<L3<<endl;
	return 0;
}

template<class T>
void Merge(const LinkList<T> &L1, const LinkList<T> &L2, LinkList<T>&L3)
{
	LinkNode<T>*p = L3.head;

	LinkNode<T>*m = L1.head->next;
	LinkNode<T>*n = L2.head->next;
	for (int i = 0; i < L1.GetLength() + L2.GetLength(); i++)
	{
		if (m!=NULL&&n!=NULL)
		{
			if (m->data<n->data)
			{
				LinkNode<T>*newNode = new LinkNode<T>;
				newNode->data = m->data;
				p->next = newNode;
				p = newNode;
				m = m->next;
			}
			else{
				LinkNode<T>*newNode = new LinkNode<T>;
				newNode->data = n->data;
				p->next = newNode;
				p = newNode;
				n = n->next;
			}
		}else if (m!=NULL)
		{
			LinkNode<T>*newNode = new LinkNode<T>;
			newNode->data = m->data;
			p->next = newNode;
			p = newNode;
			m = m->next;
		}
		else{
			LinkNode<T>*newNode = new LinkNode<T>;
			newNode->data = n->data;
			p->next = newNode;
			p = newNode;
			n = n->next;
		}
	}
}

这么改

C和C++完整教程：https://blog.csdn.net/it_xiangqiang/category_10581430.html
C和C++算法完整教程：https://blog.csdn.net/it_xiangqiang/category_10768339.html