Binary search tree in c (but has problem in delete function)

binary_search_tree.c

#include <stdio.h>
#include <stdlib.h>

struct node {
	int data;
	struct node* left;
	struct node* right;
};
struct node* root = NULL;


struct node* insert(struct node* temp, int data) {

	struct node* new_node  = (struct node*) malloc(sizeof(struct node));
	new_node -> right = NULL;
	new_node -> left = NULL;
	new_node -> data = data;
	if (temp == NULL) {
		temp = new_node;
		//printf("First element\n");
		return temp;
	}
	else {
		while (temp != NULL) {
			if (temp -> data > new_node -> data) {
				if (temp -> left == NULL) {
					temp -> left = new_node;
				//	printf("Adding to left side of the root\n");
					break;
				}
				else
					temp = temp -> left;
			}
			else {
				if (temp -> right == NULL) {
				//	printf("Adding to the right side of the root\n");
					temp -> right = new_node;
					break;
				}
				else 
					temp = temp -> right;
			}
		}
		
	}	
}

int search(struct node* tree, int key) {

	while (tree != NULL) {

		if (tree -> data == key) {
			printf("Element found\n");
			return -1;
		}
		if (tree -> data > key) {
			tree = tree -> left;
		} 
		else {
			tree = tree -> right;
		}
	}
	printf("Element not found\n");

}

void delete(struct node* tree, int key) {
		
	struct node* prev = NULL;
	while (tree != NULL) {
		if (tree -> data == key) {
			if (tree -> right == NULL && tree -> left == NULL) {
					// This doesn't work, why?
				if (prev -> left -> data ==  tree -> data) {
				
					printf("YES\n"); 
					printf("prev -> left -> data = %d\n", prev -> left -> data);
					printf("tree -> data = %d\n", tree -> data);
					printf("{Before}prev -> left = %d\n", prev -> left);
					prev -> left == NULL;
					free(tree);
					printf("{After} prev -> left = %d\n", prev -> left);
					printf("{After} prev -> left -> data = %d\n", prev -> left -> data);
					prev -> right == NULL;
					break;
				}
				// This works, Why?	
				//free(tree);
				//printf("Prev -> data = %d\n", prev -> data);
				//printf("%d\n", prev -> left);
				//prev -> left = NULL;
				//printf("%d\n", prev -> left);
				//break;
			}
		}
		if(tree -> data > key) {
		prev = tree;
		tree = tree -> left;
		}
		else {
		prev = tree;
		tree = tree -> right;
		}
	}
}

int main(void) {

	root = insert(root, 10);
	insert(root, 20);
	insert(root, 30);
	insert(root, 25);
	insert(root, 5);
	insert(root, 1);
	//search(root, 5);
	//search(root, 20);
	//search(root, 30);
	//search(root, 25);
	search(root, 1);
	delete(root, 1);
	search(root, 1);
	//search(root, 5);
//	delete(root, 5);
	//search(root, 5);
	
	return 0;
}