Python Binary Search Tree: Check a binary tree is valid or not

Last update on April 01 2025 13:16:58 (UTC/GMT +8 hours)

3. Validate BST

Write a Python program to check whether a given binary tree is a valid binary search tree (BST) or not.

Let a binary search tree (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:
    2
   / \
  1   3
Binary tree [2,1,3], return true.
Example 2:
    1
   / \
  2   3
Binary tree [1,2,3], return false.

Sample Solution:

Python Code:

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def is_BST(root):
    stack = []
    prev = None
    
    while root or stack:
        while root:
            stack.append(root)
            root = root.left
        root = stack.pop()
        if prev and root.val <= prev.val:
            return False
        prev = root
        root = root.right
    return True

root = TreeNode(2)  
root.left = TreeNode(1)  
root.right = TreeNode(3) 
 
result = is_BST(root)
print(result)

root = TreeNode(1)  
root.left = TreeNode(2)  
root.right = TreeNode(3) 
 
result = is_BST(root)
print(result)

Sample Output:

True
False

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Python program to check if a binary tree is a valid BST using in-order traversal and compare the resulting list with its sorted version.
• Write a Python script to recursively validate a BST by ensuring each node’s value lies within a valid range, then test it on a tree with duplicate values.
• Write a Python program to implement an iterative BST validation algorithm using a stack and then output whether the tree is valid.
• Write a Python function to determine if a binary tree is a valid BST and also print the minimum and maximum values found in each subtree.

Python Code Editor:

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