Problem URL: Validate binary search tree

We will run a DFS, and each iteration we will compare the value of the node with a left and right value, which we will pass along each iteration. For root, this values will be negative and positive infinity.

# Definition for a binary tree node.
# 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:
        def isValid(node, left, right):
            if not node:
                return True
            if not (node.val > left and node.val < right):
                return False
            return isValid(node.left, left, node.val) and isValid(node.right, node.val, right)

        return isValid(root, -math.inf, math.inf)

Time Complexity: O(n)
Space Complexity: O(1)