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    <p>I see four errors in your code.</p> <ol> <li><p>First, your check for null children is backwards in <code>tree_min</code>. That is, you're checking if <code>node.right</code> exists before accessing <code>node.left</code>, and vise versa.</p></li> <li><p>Second, <code>tree.min</code> returns negative infinity when called on a leaf node. You need to use positive infinity in the min calculation (negative infinity is correct in the max version).</p></li> <li><p>Third, you have a logic error within <code>verify</code>, as it unconditionally calls <code>tree_min</code> or <code>tree_max</code> and itself on it's child nodes, even if one or both of them are <code>None</code>. I suggest making all the functions handle being passed <code>None</code>, rather than relying on the caller to do the right thing. This also simplifies the <code>min</code> and <code>max</code> code a bit!</p></li> <li><p>Lastly, you're doing your comparisons on <code>node.value</code>, which is the string you're giving each node. I suspect you want to be comparing using <code>node.key</code> instead. Comparing a float (like <code>float("-inf")</code>) to a string (like <code>"ten"</code>) is an error in Python 3, and even in Python 2 where it is legal, it probably doesn't work like you would expect.</p></li> </ol> <p>With those issues fixed, I get expected results when I create valid and invalid trees. Your two examples are both invalid though, so if you were using them to test, you will always get a <code>False</code> result.</p> <p>Finally, a couple of minor style issues (that aren't bugs, but still things that could be improved). Python supports chained comparisons, so you can simplify your first <code>if</code> statement in <code>verify</code> to <code>tree_max(node.left) &lt;= node.key &lt;= tree_min(node.right)</code>. You can further simplify that part of the code by connecting the checks with <code>and</code> rather than nesting an additional <code>if</code> statement.</p> <p>Here's a version of your code that works for me (using Python 3, though I think it is all backwards compatible to Python 2):</p> <pre><code>class Node: def __init__(self, k, val): self.key = k self.value = val self.left = None self.right = None def tree_max(node): if not node: return float("-inf") maxleft = tree_max(node.left) maxright = tree_max(node.right) return max(node.key, maxleft, maxright) def tree_min(node): if not node: return float("inf") minleft = tree_min(node.left) minright = tree_min(node.right) return min(node.key, minleft, minright) def verify(node): if not node: return True if (tree_max(node.left) &lt;= node.key &lt;= tree_min(node.right) and verify(node.left) and verify(node.right)): return True else: return False root= Node(10, "Hello") root.left = Node(5, "Five") root.right= Node(30, "Thirty") print(verify(root)) # prints True, since this tree is valid root = Node(10, "Ten") root.right = Node(20, "Twenty") root.left = Node(5, "Five") root.left.right = Node(15, "Fifteen") print(verify(root)) # prints False, since 15 is to the left of 10 </code></pre>
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    1. COThank You very much...Great explanation.
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      1. USkoala421
 

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