比如这个 递归二叉树

#include <stdio.h>
#include <stdlib.h>
typedef int datatype;
typedef struct node
{   
    datatype data;
    struct node *lchild;
    struct node *rchild;
}BINTNODE;           
typedef BINTNODE * BINTREE;   /*把BINTNODE的指针类型命名为BINTREE*/               /*定义二叉链表指针类型*/

void createbintree(BINTNODE **t)   /*这里为什么要二级指针*/
{   
    int a;
     scanf("%d",&a);
     if(a==0)
         *t=NULL;
     else
     {  
         *t=(BINTNODE*)malloc(sizeof(BINTNODE));
     (*t)->data=a;
         createbintree(&(*t)->lchild);
         createbintree(&(*t)->rchild);
     }
}

void preorder(BINTREE T)         /*而这里不用二级指针~?*/           /*先序遍历序列*/
{
    if (T)                                          /*如果二叉树非空*/
    {
        printf("%d ",T->data);               /*访问根结点*/
        preorder(T->lchild);
        preorder(T->rchild);
    }
}

error(char *message)
{
    printf(stderr,"error:%s\n",message);
    exit(1);
}

main() /*主函数*/
{   BINTREE t=NULL;
    printf("\nPlease input nodes of BINTREE : ");
    createbintree(&t);               /*这里传的是地址 */
    printf("ok!\nthe preorder is:");
    preorder(t);                     /*那这里传的是啥~?*/
   
    getch();
}