이진트리의 높이를 출력하는 프로그램을 만들어 보았는데 질문이 있어 글을 올립니다!

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#include <time.h>
#define M 1000000 // 백만
 
struct record {
	int num;
	struct record * left;
	struct record * right;
};
 
static struct record pool[N];
struct record * top = pool; 
 
struct record * data = NULL;
 
 
struct record * new_node()
{
	struct record *r;
 
	if (top == NULL)
		return NULL;
 
	r = top;
	top = r->right;
	return r;
}
 
void free_node(struct record *r)
{
	r->right = top;  
	top = r;
}
 
void add(int addnum)
{
	struct record* p = data;
	struct record** g = &data;
 
 
	while (p != NULL) {
		if (addnum < p->num) {
			g = &(p->left);
			p = p->left;
		}
		else {
			g = &(p->right);
			p = p->right;
		}
	}
 
	struct record* newNode = new_node();
	if (newNode == NULL) {
		printf("Can not add\n");
		return;
	}
	newNode->num = addnum;
 
 
	newNode->left = NULL;
	newNode->right = NULL;
 
	*g = newNode;
}
int height(struct record *t)
{
	if (t == NULL)
		return -1;
	else
		return (((height(t->left)>height(t->right)) ? height(t->left) : height(t->right)) + 1);
 
}
void print_height()
{
	printf("BST's height is %d.\n", height(data));
}
int main()
{
	time_t t;
	int i, j;
	srand((unsigned)time(&t)); 
 
	struct record *r = pool;
	struct record *s;
	pool[M - 1].right = NULL;
 
	for (j = 0; j < 100; j++) {
		for (i = 0; i < M; i++) {
			s = r++;
			s->right = r;
		}
 
		for (i = 0; i < M; i++) {
			add(rand());
		}
		print_height();
		free_node(data);
	}
 
}