98. Validate Binary Search Tree

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        def inorder(node: Optional[TreeNode]) -> Iterator[int]:
            if node:
                yield from inorder(node.left)
                yield node.val
                yield from inorder(node.right)

        return all(a < b for a, b in pairwise(inorder(root)))

class Solution:
    def isValidBST(
        self, node: Optional[TreeNode], low: int = -inf, high: int = inf
    ) -> bool:
        if not node:
            return True

        if node.val <= low or node.val >= high:
            return False

        return self.isValidBST(node.left, low, node.val) and self.isValidBST(
            node.right, node.val, high
        )

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        stack = []
        node = root

        pre = -inf
        while stack or node:
            while node:
                stack.append(node)
                node = node.left
            node = stack.pop()
            if node.val <= pre:
                return False
            pre = node.val
            node = node.right

        return True