How to create AVL - c

Question

I enter several numbers(2,1,4,5,9,3,6,7),after I enter the number '3', there something wrong,the function can not return correctly.
#include <stdio.h>
#include <stdlib.h>
typedef struct AVLNode
{
int data;
int height;
struct AVLNode *LChild;
struct AVLNode *RChild;
}*AVLTree;
typedef struct AVLNode *Position;
static int Height(Position T)
{
if (T == NULL)
return -1;
else
return T->height;
}
static Position SingleLeft(Position k2)
{
Position k1;
k1 = k2->LChild;
k2->LChild = k1->RChild;
k1->RChild = k2;
k2->height = max(Height(k2->LChild), Height(k2->RChild)) + 1;
k1->height = max(Height(k1->LChild), Height(k1->RChild)) + 1;
return k1;
}
static Position SingleRight(Position k1)
{
Position k2;
k2 = k1->RChild;
k1->RChild = k2->LChild;
k2->LChild = k1;
k1->height = max(Height(k1->LChild), Height(k1->RChild)) + 1;
k2->height = max(Height(k2->LChild), Height(k2->RChild)) + 1;
return k2;
}
static Position DoubleLeft(Position k3)
{
k3->LChild = SingleRight(k3->LChild);
return SingleLeft(k3);
}
static Position DoubleRight(Position k1)
{
k1->RChild = SingleLeft(k1->RChild);
return SingleRight(k1);
}
void PrePrint(AVLTree T)
{
if (T != NULL)
{
printf("%d ", T->data);
PrePrint(T->LChild);
PrePrint(T->RChild);
}
}
AVLTree Insert(int x, AVLTree T)
{
if (T == NULL)
{
T = (AVLTree)malloc(sizeof(struct AVLNode));
T->data = x;
T->LChild = T->RChild = NULL;
}
else if (x < T->data)
{
T->LChild = Insert(x, T->LChild);
if (Height(T->LChild) - Height(T->RChild) == 2)
{
if (x<T->LChild->data)
T = SingleLeft(T);
else
T = DoubleLeft(T);
}
}
else if (x > T->data)
{
T->RChild = Insert(x, T->RChild);
if (Height(T->RChild) - Height(T->LChild) == 2)
{
if (x>T->RChild->data)
T = SingleRight(T);
else
T = DoubleRight(T);
}
}
T->height = max(Height(T->LChild), Height(T->RChild)) + 1;
return T;
}
I think there is something wrong in my main function, I think I shouldn't write
T=(AVLTree)malloc(sizeof(struct AVLNode));
T->LChild = T->RChild = NULL;
those code in mian function, I try to add a 'Init' function, but it doesn't work. It always said "'T' is being used without initialized"
int main()
{
AVLTree T;
T=(AVLTree)malloc(sizeof(struct AVLNode));I think there is wrong
T->LChild = T->RChild = NULL;
int x;
printf("please enter the data(0 to quit):");
scanf("%d", &x);
T->data = x;
while (x != 0)
{
Insert(x, T);
printf("enter a number(0 to quit):");
scanf("%d", &x);
}
PrePrint(T);
}

Answer 1

When your insertion makes a new root node, this fact is not propagated back to main in any way. The value of T inside the Insert function changes, but main has its own variable called T that isn't changed, and that's the one that you then use to print out the tree.
I notice that your Insert function returns an AVLTree, but when main calls it it doesn't do anything with the return value.
(This is not the only thing that's amiss in your code, but it would be a good place to start.)