单链表操作：逆序、合并

备注：常常看到有关单链表的逆序、合并问题，于是尝试用非递归及递归方法实现。

一、问题描述：

对给定没有头结点的单链表link，如何将其逆序，仍用link表示逆序结果。对没有头结点的各结点按数据域升序排列的单链表link1和link2，如何将其合并，使合并后的单链表也为升序，而单链表link1和link2销毁。设链表结点定义如下：

typedef struct NODE{
	int data;
	struct NODE *next;
}NODE;

二、链表逆序：

1. 非递归实现：

NODE *reverse_link(NODE *&link){
	//link: pointer of first node of the origin link, 
	//	after the reverse process is done points to the first node of the reversed link;
	//return: pointer of the first node of the reversed link;
	if(link!=NULL&&link->next!=NULL){
		NODE *p1=link->next;
		NODE *p2=p1->next;
		link->next=NULL;
		while(p2!=NULL){
			p1->next=link;
			link=p1;
			p1=p2;
			p2=p2->next;
		}
		p1->next=link;
		link=p1;
	}
	return link;
}

2. 递归实现：

NODE *recursive_reverse_link(NODE *&link){
	//link: pointer of the first node of the origin link, 
	//	after the reverse process is done points to the first node of the reversed link;
	//return: pointer of the last node of the reversed link;
	if(link==NULL){
		return NULL;
	}else if(link->next==NULL){
		return link;
	}else{
		NODE *temp1=link;
		link=link->next;
		NODE *temp2=recursive_reverse_link(link);
		temp1->next=NULL;
		temp2->next=temp1;
		return temp1;
	}
}

三、链表合并：

1. 非递归实现：

NODE *merge_link(NODE *&link1,NODE *&link2){
	//link1: pointer of first node of the origin link1, after the merging process set to NULL;
	//link2: pointer of first node of the origin link2, after the merging process set to NULL;
	//return: pointer of first node of the merged link;
	NODE *link;
	if(link1==NULL){
		link=link2;
	}else if(link2==NULL){
		link=link1;
	}else{
		if(link1->data<link2->data){
			link=link1;
			link1=link1->next;
		}else{
			link=link2;
			link2=link2->next;
		}
		NODE *curr=link;
		while(link1!=NULL&&link2!=NULL){
			if(link1->data<link2->data){
				curr->next=link1;
				curr=link1;
				link1=link1->next;
			}else{
				curr->next=link2;
				curr=link2;
				link2=link2->next;
			}
		}
		if(link1==NULL){
			curr->next=link2;
		}
		if(link2==NULL){
			curr->next=link1;
		}	
	}
	link1=link2=NULL;
	return link;
}

2. 递归实现：

NODE *recursive_merge_link(NODE *&link1,NODE *&link2){
	NODE *link;
	if(link1==NULL){
		link=link2;
		link2=NULL;
	}else if(link2==NULL){
		link=link1;
		link1=NULL;
	}else{
		if(link1->data<link2->data){
			link=link1;
			link1=link1->next;
			link->next=recursive_merge_link(link1,link2);
		}else{
			link=link2;
			link2=link2->next;
			link->next=recursive_merge_link(link1,link2);
		}
	}
	return link;
}

四、测试：

#include<iostream>
#include<cstdlib>
#include<cstdio>

using namespace std;

typedef struct NODE{
	int data;
	struct NODE *next;
}NODE;

void create_link(NODE *&link,int size);
void destroy_link(NODE *&link);
void print_link(const NODE *link);
NODE *reverse_link(NODE *&link);
NODE *recursive_reverse_link(NODE *&link);
NODE *merge_link(NODE *&link1,NODE *&link2);
NODE *recursive_merge_link(NODE *&link1,NODE *&link2);

int main(int argc,char *argv[]){
	NODE *link1=NULL;
	NODE *link2=NULL;
	NODE *link_m=NULL;
	int size=5;
	create_link(link1,size);
	create_link(link2,size);

	print_link(link1);
	print_link(link2);

	reverse_link(link1);
	recursive_reverse_link(link2);

	print_link(link1);
	print_link(link2);

	reverse_link(link1);
	recursive_reverse_link(link2);
	print_link(link1);
	print_link(link2);
	link_m=recursive_merge_link(link1,link2);
	//link_m=merge_link(link1,link2);
	print_link(link_m);
	print_link(link1);
	print_link(link2);

	destroy_link(link1);
	destroy_link(link2);
	destroy_link(link_m);
}

void create_link(NODE *&link,int size){
	if(size<1){
		return;
	}
	
	if((link=(NODE*)malloc(sizeof(NODE)))!=NULL){
		link->data=size;
		link->next=NULL;
	}else{
		exit(0);
	}
	NODE *temp=NULL;
	while(--size>0){
		if((temp=(NODE*)malloc(sizeof(NODE)))!=NULL){
			temp->data=size;
			temp->next=link;
			link=temp;
		}else{
			destroy_link(link);
			exit(0);
		}
	}
	return;
}

void destroy_link(NODE *&link){
	NODE *temp=NULL;
	while(link!=NULL){
		temp=link;
		link=link->next;
		free(temp);
	}
	return;
}

void print_link(const NODE *link){
	if(link!=NULL){
		cout<<link->data;
		while((link=link->next)!=NULL){
			cout<<"->"<<link->data;
		}
		cout<<endl;
	}
	return;
}