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You are given the root of a binary tree.
Find and return the largest subtree of that tree,
which is a valid binary search tree.
Here's a starting point:
```python
class TreeNode:
def __init__(self, key):
self.left = None
self.right = None
self.key = key
def __str__(self):
# preorder traversal
answer = str(self.key)
if self.left:
answer += str(self.left)
if self.right:
answer += str(self.right)
return answer
def largest_bst_subtree(root):
# Fill this in.
# 5
# / \
# 6 7
# / / \
# 2 4 9
node = TreeNode(5)
node.left = TreeNode(6)
node.right = TreeNode(7)
node.left.left = TreeNode(2)
node.right.left = TreeNode(4)
node.right.right = TreeNode(9)
print largest_bst_subtree(node)
#749
```
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