插入双链表的详细解析

#include <stdlib.h>
#include <assert.h>
#include <stdio.h>

typedef struct NODE
{
 struct NODE *f;
 struct NODE *b;
 int    value;

}Node;

int dll_insert( register Node *rootp, int value)
{
    register Node *this;
 register Node *next;
 register Node *newnode;

 for( this = rootp; ( next = this -> f) != NULL; this = next)
 {
  if( next -> value == value)
   return 0;
  if( next -> value > value)
   break;
 }

    newnode = ( Node* )malloc( sizeof( Node ));
 if( newnode == NULL)
  return -1;
 newnode -> value = value;

    newnode -> f = next;
 this -> f = newnode;

 if( this != rootp)
  newnode -> b = this;
 else
        newnode -> b = NULL;

 if( next != NULL)
  next -> b = newnode;
 else
  rootp -> b = newnode;

 return 1;
}

int main()
{
   Node* root;
   root = ( Node* )malloc( sizeof( Node ) );//创建一个根指针
   root -> f = NULL;//root里面f中的内容为NULL
   root -> b = NULL;//root里面b中的内容为NULL
   dll_insert( root, 3 );//在一个空的双链表中插入3
   dll_insert( root, 5 );//在双链表中插入5

   printf( "%d\n", root -> f -> value );
   printf( "%d\n", root -> f -> f -> value );
   printf( "%d\n", root -> b -> value );
   printf( "%d\n", root -> b -> b -> value );

   return 0;

}