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- Interview Preparation Kit
- Trees
- Trees: Is This a Binary Search Tree?

# Trees: Is This a Binary Search Tree?

# Trees: Is This a Binary Search Tree?

For the purposes of this challenge, we define a *binary search tree* to be a *binary tree* with the following properties:

- The value of every node in a node's left subtree is
*less than*the data value of that node. - The value of every node in a node's right subtree is
*greater than*the data value of that node. - The value of every node is distinct.

For example, the image on the left below is a valid BST. The one on the right fails on several counts:

- All of the numbers on the right branch from the root are not larger than the root.

- All of the numbers on the right branch from node *5* are not larger than *5*.

- All of the numbers on the left branch from node *5* are not smaller than *5*.

- The data value *1* is repeated.

Given the root node of a binary tree, determine if it is a binary search tree.

**Function Description**

Complete the function *checkBST* in the editor below. It must return a *boolean* denoting whether or not the binary tree is a binary search tree.

checkBST has the following parameter(s):

*root*: a reference to the root node of a tree to test

**Input Format**

You are not responsible for reading any input from stdin. Hidden code stubs will assemble a binary tree and pass its root node to your function as an argument.

**Constraints**

**Output Format**

Your function must return a boolean *true* if the tree is a binary search tree. Otherwise, it must return *false*.

**Sample Input**

**Sample Output**

```
Yes
```

**Explanation**

The tree in the diagram satisfies the ordering property for a Binary Search Tree, so we print `Yes`

.