30. YA

from math import inf

def solution(root) -> bool:
    def dfs(root, min_val, max_val):
        if not root:
            return True

        if root.val <= min_val or root.val >= max_val:
            return False

        left = dfs(root.left, min_val, root.val)
        right = dfs(root.right, root.val, max_val)

        return left and right

    return dfs(root, -inf, inf)