Validate Binary Search Tree

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid BST is defined as follows:

The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [5,1,4,null,null,3,6]
Output: false
Explanation: The root node's value is 5 but its right child's value is 4.

Solution

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
        
class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        out = []

        def inorder(root):
            if not root:
                return

            inorder(root.left)
            out.append(root.val)
            inorder(root.right)

        inorder(root)
        return out == sorted(set(out))


if __name__ == '__main__':
    root = TreeNode(2)
    root.left = TreeNode(1)
    root.right = TreeNode(3)
    print(root.isValidBST(root))