用栈实现二叉树的先序遍历出错，请问是哪里有问题呢？

typedef char Tchar;

typedef struct TreeNode {
    Tchar data;
    struct TreeNode* lchild;
    struct TreeNode* rchild;
}BTNode;

typedef BTNode* STDatatype;

typedef struct Stack
{
    STDatatype* a;
    int top;
    int capacity;
}ST; 

//建立二叉树
void CreatBTree(BTNode** T) {
char ch;
scanf_s("%c", &ch, 1);
if (ch == '#') {
    *T = NULL;
}
else {
    *T = (BTNode*)malloc(sizeof(BTNode));
    if (*T == NULL) {
        return;
    }
    else {
        (*T)->data = ch;
        CreatBTree(&(*T)->lchild);
        CreatBTree(&(*T)->rchild);
    }
}
return;
}
//栈扩容
void Expend(ST* ps) {
    assert(ps);
    if (ps->top == ps->capacity)
    {
        int newCapacity = ps->capacity = 0 ? 4: ps->capacity * 2;
        STDatatype* tmp = realloc(ps->a, sizeof(STDatatype) * newCapacity);
        if (tmp == NULL) {
            printf("relloc fail");
            exit(-1);
        }
        ps->a = tmp;
        ps->capacity = newCapacity;
    }
};
//初始化栈
void STackInit(ST* ps)
{
    assert(ps);
    ps->a = NULL;
    ps->capacity = 0;
    ps->top = 0;
};
void STackDestroy(ST* ps)
{
    assert(ps);
    free(ps->a);
    ps->top = ps->capacity = 0;
    ps->a = NULL;

};
//进栈
void STackPush(ST* ps, STDatatype x)
{
    assert(ps);
    Expend(ps);
    ps->a[ps->top] = x;
    ps->top++;
};
//出栈
void STackPop(ST* ps) {
    assert(ps);
    assert(ps->top > 0);
    ps->top--;
    
}
获取栈顶
STDatatype STackTop(ST* ps)
{
    assert(ps);
    assert(ps->top > 0);
    return ps->a[ps->top - 1];
};
bool STackEmpty(ST* ps) {
    assert(ps);
    return ps->top == 0;
};

//先序遍历
void xian(BTNode* T) {
    ST s;
    STackInit(&s);
    if (T != NULL) {
        STackPush(&s, T);
        while (!(STackEmpty(&s))) {
            BTNode* front = STackTop(&s);
            printf("%c ", front->data);
            STackPop(&s);
            if (T->lchild != NULL) {
                STackPush(&s, front->lchild);
            }
            if (T->rchild != NULL) {
                STackPush(&s, front->rchild);
            }
        }
    }
}
//主函数
int main() {

    BTNode* T;
    CreatBTree(&T);
    xian(T);
return 0;
}

报错

在先序遍历函数xian()中，遍历到根节点时应该使用栈顶元素front而不是固定的根节点T来判断左右子树是否为空并将它们入栈，因为在栈中根节点不一定是最上层的元素。
修改建议：

void xian(BTNode* T) {
    ST s;
    STackInit(&s);
    if (T != NULL) {
        STackPush(&s, T);
        while (!(STackEmpty(&s))) {
            BTNode* front = STackTop(&s);
            printf("%c ", front->data);
            STackPop(&s);
            if (front->lchild != NULL) { //判断条件
                STackPush(&s, front->lchild);
            }
            if (front->rchild != NULL) { //判断条件
                STackPush(&s, front->rchild);
            }
        }
    }
}

如果答案对您有所帮助，望采纳。