Binary Search Tree in C

Question

I'm a Python guy. Learning C language and I've been trying to implement Binary Search Tree in C. I wrote down the code, and I've been trying from few hours but, not able to get the output as expected. Please help!

Please correct me.

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

typedef int ElementType;

typedef struct TreeNode {
  ElementType element;
  struct TreeNode *left, *right;
} TreeNode;

TreeNode *createTree(){
    //Create the root of tree
    TreeNode *tempNode;
    tempNode = malloc(sizeof(TreeNode));
    tempNode->element = 0;
    tempNode->left = NULL;
    tempNode->right = NULL;
    return tempNode;
}

TreeNode *createNode(ElementType X){
    //Create a new leaf node and return the pointer
    TreeNode *tempNode;
    tempNode = malloc(sizeof(TreeNode));
    tempNode->element = X;
    tempNode->left = NULL;
    tempNode->right = NULL;
    return tempNode;
}

TreeNode *insertElement(TreeNode *node, ElementType X){
    //insert element to Tree
    if(node==NULL){
        return createNode(X);
    }
    else{
        if(X < node->element){
            node->left = insertElement(node->left, X);
        }
        else if(X > node->element){
            node->right =  insertElement(node->right, X);
        }
        else if(X == node->element){
            printf("Oops! the element is already present in the tree.");
        }
    }
}

TreeNode *displayTree(TreeNode *node){
    //display the full tree
    if(node==NULL){
        return;
    }
    displayTree(node->left);
    printf("| %d ", node->element); 
    displayTree(node->right);
}

main(){
    //pointer to root of tree #2
    TreeNode *TreePtr;
    TreeNode *TreeRoot;
    TreeNode *TreeChild;

    //Create the root of tree
    TreePtr = createTree();

    TreeRoot = TreePtr;

    TreeRoot->element = 32;
    printf("%d\n",TreeRoot->element);

    insertElement(TreeRoot, 8);
    TreeChild = TreeRoot->left;
    printf("%d\n",TreeChild->element);  

    insertElement(TreeRoot, 2);
    insertElement(TreeRoot, 7);
    insertElement(TreeRoot, 42);
    insertElement(TreeRoot, 28);
    insertElement(TreeRoot, 1);
    insertElement(TreeRoot, 4);
    insertElement(TreeRoot, 5);

// the output is not as expected :(
    displayTree(TreeRoot);
}

Answer 1

The problem is in the insertion. If node is NULL you create a new node and return it. But what if the node is not NULL. You are making correct changes to the right/left subtree but you are not returning anything.

Change

if(X < node->element){
    node->left = insertElement(node->left, X);
}
else if(X > node->element){
    node->right =  insertElement(node->right, X);
}

to:

if(X < node->element){
    node->left = insertElement(node->left, X);
    return node; // add this.
}
else if(X > node->element){
    node->right =  insertElement(node->right, X);
    return node; // add this.
}

Answer 2

Your insertElement does not always return a value. This is why your recursive calls go wrong. Tell your compiler to warn you about mistakes like that (e.g., on gcc, use -Wall).

displayTree has a similar error, returning nothing when it is specified to return a TreeNode*.

main should also return a value (or you should declare it void).