freeCodeCamp/guide/english/certifications/coding-interview-prep/data-structures/check-if-binary-search-tree/index.md

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title: Check if Tree is Binary Search Tree
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# Check if Tree is Binary Search Tree

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## Solutions

<details><summary>Solution 1 (Click to Show/Hide)</summary>

```javascript
var displayTree = (tree) => console.log(JSON.stringify(tree, null, 2));
function Node(value) {
  this.value = value;
  this.left = null;
  this.right = null;
}
function BinarySearchTree() {
  this.root = null;
}
function isBinarySearchTree(tree) {
  if (tree.root == null) {
    return null;
  } else {
    let isBST = true;
    function checkTree(node) {
      if (node.left != null) {
        const left = node.left;
        if (left.value > node.value) {
          isBST = false;
        } else {
          checkTree(left);
        }
      }
      if (node.right != null) {
        const right = node.right;
        if (right.value < node.value) {
          isBST = false;
        } else {
          checkTree(right);
        }
      }
    }
    checkTree(tree.root);
    return isBST;
  }
};
```

</details>