C Programming Code Examples C > Sorting Searching Code Examples C Program to Check whether a Tree is a Binary Search Tree C Program to Check whether a Tree is a Binary Search Tree #include <stdio.h> #include <stdlib.h> struct node { int data; struct node* left; struct node* right; }; static struct node *prev = NULL; /*Function to check whether the tree is BST or not*/ int is_bst(struct node* root) { if (root) { if (!is_bst(root->left)) //moves towards the leftmost child of the tree and checks for the BST return 0; if (prev != NULL && root->data <= prev->data) return 0; prev = root; return is_bst(root->right); //moves the corresponding right child of the tree and checks for the BST } return 1; } struct node* newNode(int data) { struct node* node = (struct node*)malloc(sizeof(struct node)); node->data = data; node->left = NULL; node->right = NULL; return(node); } int main() { /* The input tree is as shown below 40 / \ 20 60 / \ \ 10 30 80 \ 90 */ struct node *root = newNode(40); root->left = newNode(20); root->right = newNode(60); root->left->left = newNode(10); root->left->right = newNode(30); root->right->right = newNode(80); root->right->right->right = newNode(90); if (is_bst(root)) printf("TREE 1 Is BST"); else printf("TREE 1 Not a BST"); prev = NULL; /* The input tree is as shown below 50 / \ 20 30 / \ 70 80 / \ \ 10 40 60 */ struct node *root1 = newNode(50); root1->left = newNode(20); root1->right = newNode(30); root1->left->left = newNode(70); root1->left->right = newNode(80); root1->left->left->right = newNode(40); root1->left->left->leftt = newNode(90); if (is_bst1(root1)) printf("TREE 2 Is BST"); else printf("TREE 2 Not a BST"); return 0; }