add problem of the day

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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
+```