看看先序创建,遍历二叉树哪错了,谢谢!?

#include<stdio.h>
typedef char TElemType;
typedef struct BiTNode
{
        TElemType data;
        struct BiTNode *lchild, *rchild;
}BiTNode, *BiTree;

/*递归创建一个二叉树*/
void createBiTree(BiTree t)
{
        char c;
        c = getchar();
        if(c == 's')
        {
                t = NULL;
        }
        else
        {
                t = (BiTree)malloc(sizeof(BiTNode));
                t -> data = c;
                createBiTree(t -> lchild);
                createBiTree(t -> rchild);
        }
}

/*先序遍历二叉树*/
void preOrder(BiTree t)
{
        if(t)
        {
            printf("%c", t -> data);
            preOrder(t -> lchild);
            preOrder(t -> rchild);
        }
        else
                printf("this is a empty tree!");
}
int main()
{
        BiTree t = NULL;
        clrscr();
        printf("create a tree:");
        createBiTree(t);
        printf("preOrder the tree:");
        preOrder(t);
        getch();
        return 0;
}