双向链表实现大整数的乘法

参考代码:

#include<stdio.h>
#include<string.h>
#include<stdlib.h> 
struct lennum{
	int num;
	struct lennum *next;
	struct lennum *back; 
};
struct lennum* create(char *num);
struct lennum* add(struct lennum *head1,struct lennum *head2);
struct lennum* times(struct lennum *head1,struct lennum *head2);
struct lennum *copylennum(struct lennum *head,int l);
void adddigitalprocess(struct lennum *head);
void printflennum(struct lennum *head);

int main()
{
	char strnum1[300],strnum2[300];
	scanf("%s\n",strnum1);
	scanf("%s",strnum2);
	// 记录大整数 从个位数开始存储 
	struct lennum *head1=NULL,*head2=NULL;
	head1=create(strnum1);
	head2=create(strnum2);
	// 乘法
	struct lennum *timesnum=NULL;
	timesnum = times(head1,head2);
	// 输出
	printflennum(timesnum);
}

struct lennum *create(char *num)
{
	// 将字符串数字转成一维数组进行存储  下标0对应个位数  下标1对应十位数 
	struct lennum *head=NULL,*p=NULL,*q=NULL;
	for(int i=strlen(num)-1;i>=0;i--)
		if(i==strlen(num)-1)
		{
			q = (struct lennum *)malloc(sizeof(struct lennum));
			q->num = num[i]-'0';
			q->back=NULL;
			q->next=NULL;
			head=p=q;
		}
		else
		{
			q = (struct lennum *)malloc(sizeof(struct lennum));
			q->num = num[i]-'0';
			q->back=NULL;
			q->next=NULL;
			p->next=q;
			q->back=p;
			p=q;	
		}
	return head;
}

struct lennum *add(struct lennum *head1,struct lennum *head2)
{
	int l1=0,l2=0;
	struct lennum *p=NULL,*q=NULL;
	// 计算长度
	p=head1;
	while(p!=NULL)
	{
		l1++;
		p=p->next;	
	}
	p=head2;
	while(p!=NULL)
	{
		l2++;
		p=p->next;	
	}
	// 短的整数加到长的整数上去  temphead复制最长的链表
	struct lennum *temphead=NULL;
	if(l1>l2)
	{	
		temphead=copylennum(head1,0); 
		q=head2;
	}
	else
	{	
		temphead=copylennum(head2,0); 
		q=head1;
	}
	p=temphead;
	while(q!=NULL)
	{
		p->num=p->num+q->num;
		p=p->next;
		q=q->next;
	}
	return temphead;
}

struct lennum *times(struct lennum *head1,struct lennum *head2)
{
	// 用head1的数字去乘head2
	char beginnum[2]={'0','\0'};
	struct lennum *p=NULL,*q=NULL;
	struct lennum *temphead=NULL,*sum=create(beginnum);  	// 用sum存储结果
	int l=0;
	while(head1!=NULL)
	{
		// 在head2链表前面生成l个0
		temphead = copylennum(head2,l);
		// 乘法
		p=temphead;
		while(p!=NULL)
		{
			p->num=p->num*head1->num;
			p=p->next;
		}
		// 加法 
		p=sum;     // 用于释放原sum
		sum = add(sum,temphead);
		while(p->next!=NULL)
			p=p->next;
		while(p->back!=NULL)
		{
			q=p->back;
			free(p);
			p=q;
		}
		free(p);
		// 进位
		adddigitalprocess(sum);
		// 释放temphead
		p=temphead;
		while(p->next!=NULL)
			p=p->next;
		while(p->back!=NULL)
		{
			q=p->back;
			free(p);
			p=q;
		}
		free(p);
		temphead=NULL;
		// 下一位数进行乘法 
		head1=head1->next;
		l++;
	}
	return sum;
}

struct lennum *copylennum(struct lennum *head,int l)
{
	// 复制链表head
	struct lennum *p=NULL,*q=NULL,*temphead=NULL;
	while(head!=NULL)
	{
		if(temphead==NULL)
		{
			q = (struct lennum *)malloc(sizeof(struct lennum));
			q->num = head->num;
			q->back=NULL;
			q->next=NULL;
			temphead=p=q;
		}
		else
		{
			q = (struct lennum *)malloc(sizeof(struct lennum));
			q->num = head->num;
			q->back=NULL;
			q->next=NULL;
			p->next=q;
			q->back=p;
			p=q;
		}
		head=head->next;
	}
	// 在链表temphead的前方添加l个0
	for(int i=1;i<=l;i++)
	{
		q = (struct lennum *)malloc(sizeof(struct lennum));
		q->num = 0;
		q->back=NULL;
		q->next=NULL;
		q->next=temphead;
		temphead->back=q;
		temphead=q;
	}
	return temphead;
} 

void adddigitalprocess(struct lennum *head)
{
	// 加法进位处理
	struct lennum *p=NULL,*q=NULL;
	p=head; 
	while(p->next!=NULL)
	{
		p->next->num=p->next->num + p->num/10;
		p->num=p->num%10;
		p=p->next;
	}
	// 最高位进位
	while(p->num>=10)
	{
		q = (struct lennum *)malloc(sizeof(struct lennum));
		q->num = p->num/10;
		q->back=NULL;
		q->next=NULL;
		p->next=q;
		q->back=p;
		p->num=p->num%10;
		p=q;	
	}
}

void printflennum(struct lennum *head)
{
	// 输出 
	struct lennum *p=NULL;
	p=head;
	while(p->next!=NULL)
		p=p->next;
	while(p!=NULL)
	{
		printf("%d",p->num);
		p=p->back;
	}
}