problem in creating a tree

Hi, I encountered some problems in creating a tree and using preorder algorithm to travel it.

Code:

// Create a binary search tree

#include<iostream>
struct TreeNode;
typedef struct TreeNode *PtrToNode;
typedef struct TreeNode *Tree;
using namespace std;

struct TreeNode {
	int data;
	PtrToNode left;
	PtrToNode right;
};

// insert a new node into a binary search tree
// using a recursive algorithm
void insert(Tree tree, int item) {
	if( tree == 0 ) {
		tree = (Tree)malloc(sizeof(struct TreeNode));
		if( tree == 0 ) {
			cout << "Fatal error!" << endl;
			exit(1);
		}
		else {
			tree->data = item;
			tree->left = tree->right = 0;
		}
	}

	else if( item < tree->data )
		insert( tree->left, item);
	else if( item > tree->data )
		insert( tree->right, item );

}

// create a new binary search tree
void createTree( Tree tree, int a[], int n ) {
	for( int i = 0; i < n; i++ )
		insert( tree, a[i] );
}

void preorder( Tree tree ) {
	if( tree != 0 ) {
		cout << tree ->data << ' ';
		preorder( tree->left );
		preorder( tree->right );
	}
}


int main(void) {
	const int maxsize = 5;
	int a[maxsize] = {5,3,9,1,10};
	Tree T = 0;
	createTree(T,a,maxsize);
	
	preorder( T );

	return 0;
}

so why there is no output to the screen??
thanks for your help~~~

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problem in creating a tree