<p>Given the <code>root</code> of a binary tree, <em>determine if it is a valid binary search tree (BST)</em>.</p>

<p>A <strong>valid BST</strong> is defined as follows:</p>

<ul>
	<li>The left <span data-keyword="subtree">subtree</span> of a node contains only nodes with keys <strong>less than</strong> the node&#39;s key.</li>
	<li>The right subtree of a node contains only nodes with keys <strong>greater than</strong> the node&#39;s key.</li>
	<li>Both the left and right subtrees must also be binary search trees.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree1.jpg" style="width: 302px; height: 182px;" />
<pre>
<strong>Input:</strong> root = [2,1,3]
<strong>Output:</strong> true
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/12/01/tree2.jpg" style="width: 422px; height: 292px;" />
<pre>
<strong>Input:</strong> root = [5,1,4,null,null,3,6]
<strong>Output:</strong> false
<strong>Explanation:</strong> The root node&#39;s value is 5 but its right child&#39;s value is 4.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 10<sup>4</sup>]</code>.</li>
	<li><code>-2<sup>31</sup> &lt;= Node.val &lt;= 2<sup>31</sup> - 1</code></li>
</ul>

# 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
def is_sorted(lst):
    for i in range(len(lst) - 1):
        if lst[i] >= lst[i + 1]:
            return False
    return True
class Solution:
    def isValidBST(self, root: Optional[TreeNode]) -> bool:
        L = []
        def dfs(node):
            if not node: return
            dfs(node.left)
            L.append(node.val)
            dfs(node.right)
        dfs(root)
        return is_sorted(L)

        
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