levelorder

#include "stdio.h"
#include "stdlib.h"
#include "conio.h"

typedef struct tree_node{
    int value;
    struct tree_node* l;
    struct tree_node* r;
} tree_node;

typedef struct QUEUE_ELEM{
    tree_node* elem;
    struct QUEUE_ELEM* next;
} queue_node;

void put_to_end(queue_node* queue_root, queue_node* queue_elem){
    while (queue_root->next != NULL){
        queue_root = queue_root->next;
    }
    queue_root->next = queue_elem;
}

queue_node* create_queue(queue_node* queue_root, tree_node* tree_elem){
    queue_node* new_elem = (queue_node*)malloc(sizeof(queue_node));
    new_elem->elem = tree_elem;
    new_elem->next = NULL;
    if(queue_root == NULL){
        return new_elem;
    } else {
        put_to_end(queue_root, new_elem);
        return queue_root;
    }
}

queue_node* remove_from_head(queue_node* queue_root){
    queue_node* new_queue_head = queue_root->next;
    free(queue_root);
    return new_queue_head;
}

tree_node* create_tree(tree_node* p, int key){
    if(p == NULL){
        p = (tree_node*)malloc(sizeof(tree_node));
        p->value = key;
        p->l = NULL;
        p->r = NULL;
    } else {
        if (p->value < key){
            p->r = create_tree(p->r, key);
        } else {
            p->l = create_tree(p->l, key);
        }
    }
    return p;
}

tree_node* read_file(tree_node *root){
    FILE* fp;
    fp = fopen("input", "r");
    while(1){
        int key, is_the_end = fscanf(fp, "%d", &key);
        if (is_the_end != -1){
            printf("%d\n", key);
            root = create_tree(root, key);
        } else {break;}
    }
    fclose(fp);
    return root;
}

void levelorder(tree_node* root){
    queue_node* QUEUE = NULL;
    printf("%d ", root->value);
    if(root->l != NULL) QUEUE = create_queue(QUEUE, root->l);
    if(root->r != NULL) QUEUE = create_queue(QUEUE, root->r);
    while(QUEUE != NULL){
        tree_node* next_elem = QUEUE->elem;
        printf("%d ", next_elem->value);
        QUEUE = remove_from_head(QUEUE);
        if(next_elem->l != NULL) QUEUE = create_queue(QUEUE, next_elem->l);
        if(next_elem->r != NULL) QUEUE = create_queue(QUEUE, next_elem->r);
    }
}

int main(){
    tree_node* root = NULL;
    root = read_file(root);
    levelorder(root);
    _getch();
    return 0;
}