Tree elements not displaying in correct order

Question

I am trying to build a simple tree program and traverse it in inorder,preorder and postorder formats. The code I am using is this :

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

struct node
{
    int info;
    struct node* lchild;
    struct node* rchild;
};
typedef struct node* NODE;

NODE getNode()
{
    NODE temp;
    temp = (NODE)malloc(sizeof(struct node));
    return temp;
}

NODE insert(NODE root,int item)
{
    NODE temp,prev,cur;
    temp = getNode();
    temp->info = item;
    temp->lchild = NULL;
    temp->rchild = NULL;
    if(root==NULL)
    {return temp;}
    prev = NULL;
    cur = root;
    while(cur!=NULL)
    {
        prev = cur;
        if(item<cur->info)
        {
            cur = cur->lchild;
        }
        else if(item>cur->info)
        {
            cur = cur->rchild;
        }
    }
    if(item<prev->info)
    {
        prev->lchild = temp;
    }
    else if(item>prev->info)
    {
        prev->rchild = temp;
    }
    return root;
}

void inorderTraversal(NODE root)
{
    if(root!=NULL)
    {
        inorderTraversal(root->lchild);
        printf("%d\t",root->info);
        inorderTraversal(root->rchild);
    }
    else
    {
        return;
    }
}

void preorderTraversal(NODE root)
{
    if(root!=NULL)
    {
        printf("%d\t",root->info);
        preorderTraversal(root->lchild);
        preorderTraversal(root->rchild);
    }
    else
    {
        return;
    }
}

void postorderTraversal(NODE root)
{
    if(root!=NULL)
    {
        postorderTraversal(root->lchild);
        postorderTraversal(root->rchild);
        printf("%d\t",root->info);
    }
    else
    {
        return;
    }
}





int main() {

    NODE root;
    root = getNode();
    root = NULL;
    root = insert(root,10);
    root = insert(root,20);
    root = insert(root,15);
    printf("Root is: %d\n",root->info);
    printf("Inorder Traversal is :");
    inorderTraversal(root);
    printf("\n");
    printf("Preorder traversal is :");
    preorderTraversal(root);
    printf("\n");
    printf("Postorder traversal is :");
    postorderTraversal(root);
    printf("\n");
    return 0;
}

and the output I get is this :

Root is: 10 Inorder Traversal is :10 15 20
Preorder traversal is :10 20 15
Postorder traversal is :15 20 10

ie 10 is assigned as root, when it should ideally be 15 with 10 on the left and 15 on the right. What is the error here and how do I fix this ?

Thanks !

Answer 1

The tree structure you are ending up is something like below

                    10
                      \
                       20
                     /
                   15

The order of traversal will be as below: In-order: 10 15 20 Pre-order: 10 20 15 Post-order: 15 20 10

BST does maintain the property that elements in the lef-subtree are smaller than the elements in the right sub-tree. But, they don't do self-balancing like AVL or Red-black tree.