Tree

66. 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.

Examples

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.

Constraints

The number of nodes in the tree is in the range [1, 10^4].
-2^31 <= Node.val <= 2^31 - 1

Dada la raiz root de un arbol binario, determina si es un arbol de busqueda binaria (BST) valido.

Un BST valido se define de la siguiente manera:

El subarbol izquierdo de un nodo contiene solo nodos con claves menores que la clave del nodo.
El subarbol derecho de un nodo contiene solo nodos con claves mayores que la clave del nodo.
Tanto el subarbol izquierdo como el derecho tambien deben ser arboles de busqueda binaria.

Ejemplos

Ejemplo 1

Entrada: root = [2,1,3]
Salida: true

Ejemplo 2

Entrada: root = [5,1,4,null,null,3,6]
Salida: false
Explicacion: El valor del nodo raiz es 5, pero el valor de su hijo derecho es 4.

Restricciones

El numero de nodos en el arbol esta en el rango [1, 10^4].
-2^31 <= Node.val <= 2^31 - 1

validate_binary_search_tree.py

# Validate Binary Search Tree
# Given a binary tree, determine if it is a valid BST.
# Each node must be within an allowed range (min, max) that updates as we go deeper.

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def is_valid_bst(node, min_val=float("-inf"), max_val=float("inf")):
    # An empty node is always valid
    if not node:
        return True

    # Check if current node is within its allowed range
    if node.val <= min_val or node.val >= max_val:
        return False

    # Left child must be less than current node (update max)
    # Right child must be greater than current node (update min)
    return (is_valid_bst(node.left, min_val, node.val) and
            is_valid_bst(node.right, node.val, max_val))

# Example 1: [2,1,3] -> true
tree1 = TreeNode(2, TreeNode(1), TreeNode(3))
print(is_valid_bst(tree1))  # True

# Example 2: [5,1,4,null,null,3,6] -> false
tree2 = TreeNode(5, TreeNode(1), TreeNode(4, TreeNode(3), TreeNode(6)))
print(is_valid_bst(tree2))  # False

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