Finding the InOrder Predecessor and Successor of a node in Binary Search Tree

This program illustrates how to find the inorder successor and predecessor of a given node in a Binary Search Tree.

#include<stdio.h>

#include<stdlib.h>

struct BSTNode

{

      /*

       * this is the basic structure representing a node of the Tree

       */

    int key; // key stores the data part of the node.

    struct BSTNode *left,*right,*parent;

    /*

     * the left pointer points to the left child

     * right pointer points to the right child

     * parent points to the parent of this node.

     */

};

struct BST

{

      /*

       * Any binary search tree can be identified alone by its root.

       */

    struct BSTNode *root;

};

struct BSTNode*makeNode(int key)

{

    struct BSTNode*newNode;

    newNode=(struct BSTNode*)malloc(sizeof(struct BSTNode));

    newNode->key=key;

    newNode->left=newNode->right=newNode->parent=NULL;

    return newNode;

}

void inorderTraversal(struct BSTNode*root)

{

    if(root==NULL)

        return;

    inorderTraversal(root->left);

    printf("%d ",root->key);

    inorderTraversal(root->right);

}

void BSTInsert(struct BST*T,struct BSTNode*newNode)

{

    int direction;

    struct BSTNode*currentNode,*previousNode;

    if(T->root==NULL)

        T->root=newNode;

    else

    {

        currentNode=T->root;

        while(currentNode!=NULL)

        {

            previousNode=currentNode;

            if(currentNode->key > newNode->key)

                currentNode=currentNode->left;

            else

                currentNode=currentNode->right;

        }

        if(newNode->key < previousNode->key)

            previousNode->left=newNode;

        else

            previousNode->right=newNode;

        newNode->parent=previousNode;

    }

}

struct BSTNode*treeMinimum(struct BSTNode*node)

{

   while(node->left!=NULL)

        node=node->left;

   return node;

}

struct BSTNode*inorderSuccessor(struct BSTNode*x)

{

    struct BSTNode*y;

    if(x->right!=NULL)

        return treeMinimum(x->right);

    y=x->parent;

    while(y!=NULL && x==y->right)

    {

        x=y;

        y=x->parent;

    }

    return y;

}

struct BSTNode*inorderPredecessor(struct BSTNode*x)

{

    struct BSTNode*y;

    if(x->left!=NULL)

        return treeMinimum(x->left);

    y=x->parent;

    while(y!=NULL && x==y->left)

    {

        x=y;

        y=x->parent;

    }

    return y;

}

int main()

{

    struct BSTNode*x;

    struct BST*T;

    T=(struct BST*)malloc(sizeof(struct BST*));

    T->root=NULL;

    struct BSTNode **nodeArray;

    nodeArray=(struct BSTNode**)malloc(sizeof(struct BstNode*)*10);

    nodeArray[0]=makeNode(500);

    nodeArray[1]=makeNode(200);

    nodeArray[2]=makeNode(700);

    nodeArray[3]=makeNode(100);

    nodeArray[4]=makeNode(300);

    nodeArray[5]=makeNode(400);

    nodeArray[6]=makeNode(600);

    nodeArray[7]=makeNode(0);

    nodeArray[8]=makeNode(900);

    nodeArray[9]=makeNode(800);

    BSTInsert(T,nodeArray[0]);

    BSTInsert(T,nodeArray[1]);

    BSTInsert(T,nodeArray[2]);

    BSTInsert(T,nodeArray[3]);

    BSTInsert(T,nodeArray[4]);

    BSTInsert(T,nodeArray[5]);

    BSTInsert(T,nodeArray[6]);

    BSTInsert(T,nodeArray[7]);

    BSTInsert(T,nodeArray[8]);

    BSTInsert(T,nodeArray[9]);

    x=inorderPredecessor(nodeArray[9]);

    if(x==NULL)

        printf("the inorder predecessor of the given node does not exist");

    else

        printf("%d\n",x->key);

    x=inorderSuccessor(nodeArray[4]);

    if(x==NULL)

        printf("the inorder successor of the given node does not exist");

    else

        printf("%d\n",x->key);

    return 0;

}

For complete program of Binary search Tree in C visit here and here (for program as per guidelines provided by Cormen)