Create 0098-validate-binary-search-tree.md

[docto-rin]

この問題：https://leetcode.com/problems/validate-binary-search-tree/
次の問題：https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/

[oda]

特に返り値を利用した再帰だと面倒です。

[oda]

このあたりで inorder を iterative にする話があります。
https://discord.com/channels/1084280443945353267/1200089668901937312/1201416402247098398

[oda]

一応、私が帰りがけ iterative でイメージしたものと違ったので書いておきます。

class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        stack = [([], root, [], [])]
        while stack:
            from_to, node, left, right = stack[-1]
            if not left and node.left:
                stack.append((left, node.left, [], []))
                continue
            if not right and node.right:
                stack.append((right, node.right, [], []))
                continue
            if left and left[1] >= node.val:
                return False
            if right and node.val >= right[0]:
                return False
            r = left + [node.val] + right
            from_to += [min(r), max(r)]
            stack.pop()
        return True