C++下面一段代码中的if的情况感觉不懂

Question

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct node{
    int data;
    struct node *prior;
    struct node *next;
} node;//双向链表结构体定义
int main()
{
    node *num1,*num2,*sum;
    node *create(node *n);
    int output(node *n);
    node *add_Num(node *n1,node *n2);
    num1=(node *)malloc(sizeof(node));
    num2=(node *)malloc(sizeof(node));
    printf("求两个任意长整数的和\n\n"); 
    printf("输入时每四位用逗号隔开\n\n如：123456789\n\n");
    printf("输入格式为：1,2345,6789\n\n");
    printf("输入整数1：");
    num1=create(num1);//建立第一个新结点 
    printf("输入整数2："); 
    num2=create(num2);//建立第二个新结点 
    printf("\n两整数的和为：");
    sum=add_Num(num1,num2);
    output(sum);
}//定义主函数 
int output(node *n)
{
  node *p;
  if(n->data<0) //判断输出结果的正负 
  printf("-");
  printf("%d,",n->next->data);
  p=n->next->next;
  while(p!=n->next)
  {
    if(p->data<1000) printf("0");
    if(p->data<100) printf("0");
    if(p->data<10) printf("0");
    printf("%d,",p->data);
    p=p->next;
   }
  printf("\b \b\n");
 }//控制输出的格式 
node *create(node *n)
{//创建一个双向链表 
  node *p,*t;
  int data,i;
  t=(node *)malloc(sizeof(node));
  i=scanf("%d",&data);
  if(data<0)
  n->data=-1;//如果长整数小于0则把符号保存在头结点中 
  else 
  n->data=1;//如果长整数大于0则把符号保存在头结点中 
  t->data=n->data*data;
  n->next=t;
  t->next=t;
  t->prior=t;
  n->prior=NULL;
  p=n->next;
  while(i!=0)
  {
    if(getchar()==',')//如果输入时“，”的话则执行if语句 
    i=scanf("%d",&data);
    else break;
    t=(node *)malloc(sizeof(node));
    t->data=data;
    p->next=t;
    t->prior=p;
    t->next=n->next;
    n->next->prior=t;
    p=p->next;
   }
  return(n);
 }
node *add_Num(node *n1,node *n2)
{
  node *p1,*p2,*t,*s,*sum;
  int e1,e2,carry;
  p1=n1->next->prior;
  p2=n2->next->prior;
  e1=n1->data;
  e2=n2->data;
  sum=(node *)malloc(sizeof(node));
  sum->data=1;
  sum->prior=NULL;
  s=(node *)malloc(sizeof(node));
  s->data=p1->data*e1+p2->data*e2;
  p1=p1->prior;
  p2=p2->prior;
  if(e1*e2==1)//如果两个数同号 
  {
    sum->data=e1;
    s->data=abs(s->data);
    if(s->data>=10000)//相加大于等于10000则进位 
    {
      s->data-=10000;
      carry=1;
     }
    else carry=0;//否则不进位 
   }
  else//如果异号 
    if(s->data>=0) 
    carry=0;
    else
    {
      s->data+=10000;
      carry=-1;
     }
  t=s;
  s->next=s;
  s->prior=s;
  sum->next=s;
  while(p1!=n1->next->prior&&p2!=n2->next->prior)//将链表当前指针指向表尾 
  {
    s=(node *)malloc(sizeof(node));
 s->data=p1->data*e1+p2->data*e2+sum->data*carry;
    p1=p1->prior;
    p2=p2->prior;
    if(e1*e2==1)
    {
      sum->data=e1;
      s->data=abs(s->data);
      if(s->data>=10000)
      {
        s->data-=10000;
        carry=1;
       }
      else carry=0;
     }
    else
      if(s->data>=0) carry=0;
      else
      {
        s->data+=10000;
        carry=-1;
       }
    s->next=t;
    t->prior=s;
    s->prior=sum->next;
    sum->next->next=s;
    t=t->prior;
   }

  if(p1==n1->next->prior)
    while(p2!=n2->next->prior)
    {
      s=(node *)malloc(sizeof(node));
      s->data=p2->data*e2+sum->data*carry;
      p2=p2->prior;
      if(e1*e2==1)
      {
        sum->data=e1;
        s->data=abs(s->data);
        if(s->data>=10000)
        {
          s->data-=10000;
          carry=1;
         }
        else carry=0;
       }
      else
        if(s->data>=0) carry=0;
        else
        {
          s->data+=10000;
          carry=-1;
         }
      s->next=t;
      t->prior=s;
      s->prior=sum->next;
      sum->next->next=s;
      t=t->prior;
     }
  if(p2==n2->next->prior)
    while(p1!=n1->next->prior)
    {
      s=(node *)malloc(sizeof(node));
      s->data=p1->data*e1+sum->data*carry;
      p1=p1->prior;
      if(e1*e2==1)
      {
        sum->data=e1;
        s->data=abs(s->data);
        if(s->data>=10000)
        {
          s->data-=10000;
          carry=1;
         }
        else carry=0;
       }
      else
        if(s->data>=0) carry=0;
        else
        {
          s->data+=10000;
          carry=-1;
         }
      s->next=t;
      t->prior=s;
      s->prior=sum->next;
      sum->next->next=s;
      t=t->prior;
     }
  if(carry!=0)
  {
    s=(node *)malloc(sizeof(node));
    s->data=1;
    carry=0;
    s->next=t;
    t->prior=s;
    s->prior=sum->next;
    sum->next->next=s;
    t=t->prior;
   }
  sum->next=t;
  do
  {
    if(t->data==0)
    {
      sum->next=t->next;
      t->next->prior=t->prior;
      t->prior->next=t->next;
      free(t);
     }
    else break;
    t=sum->next;
   }while(t!=sum->next->prior);
  return(sum);
 }

我自己理解加了注释但不知道对不对，if之后的分情况讨论感觉不能读懂
就是data*e1 e1*e2==1不知道是哪种情况

Answer 1

这个应该就是两个长整数的加法啊，通过链表实现每一位分别相加的