<pre name="code" class="cpp">/*全局变量，始终指向刚刚访问的结点*/
btree_node* p = NULL;

void thread_via_tree(btree_node* root)
{
 if(root != NULL)
 {
  /*p保留了其前驱元素*/
  if(p != NULL)
  {
   p->left = root;
   p = NULL;
  }
 

/*全局变量，始终指向刚刚访问的结点*/
btree_node* p = NULL;

void thread_via_tree(btree_node* root)
{
 if(root != NULL)
 {
  /*p保留了其前驱元素*/
  if(p != NULL)
  {
   p->left = root;
   p = NULL;
  }
 
  /*此节点的左孩子指针域为空，
  记录此节点的指针域准备指向其后继元素
  保留到下次进入此函数的结点，即是其后继元素*/
  if(root->left == NULL)
  {
   p = root;
  }
 
  thread_via_tree(root->left);
  thread_via_tree(root->right);
 }
}

实现代码：
void thread_via_list(btree_node* root, SeqList* list)
{
 if((root != NULL) && (list != NULL))
 {
  SeqList_Insert(list, (SeqListNode*)root, SeqList_Length(list));
  thread_via_list(root->left, list);
  thread_via_list(root->right, list);
 }
 
}