Two medium questions:\n1. https://leetcode.com/problems/merge-intervals/\n2. https://leetcode.com/problems/validate-binary-search-tree/\n\nHad a optimal solution with minor bug for 1.\nExplained the idea for 2 (implemented but did not have time to test - likely had bugs). I was too nervous and couldn\'t calm down to think.\nI had actually solved 2 only two weeks ago :(\n```\nclass Solution:\n    def isValidBST(self, root: TreeNode) -> bool:\n        \n        st = collections.deque()\n        st.append((root, -float(\'inf\'), float(\'inf\')))\n        \n        while st:\n            node, lower, upper = st.popleft()\n            if not node:\n                continue\n            \n            if not lower < node.val < upper:\n                return False\n            \n            left = node.left\n            right = node.right\n            \n            st.append((left, lower, node.val))\n            st.append((right, node.val, upper))\n        \n        return True\n```

Two medium questions:\n1. https://leetcode.com/problems/merge-intervals/\n2. https://leetcode.com/problems/validate-binary-search-tree/\n\nHad a optimal solutio