Pointer bug hunt

Questions 1 to 5 of 9

1. 1. What is the bug in this function? ``python def add_last_broken(head, v): cur = head while cur is not None: cur = cur.next cur.next = Node(v) ` Called on build(1, 2, 3), it raises AttributeError`.

   The new node is created with no value, so its next link is None and the list ends early.The method has no return value, so the caller never sees the new node.The loop checks cur instead of cur.next, so it walks past the last node and cur is None when cur.next = ... runs.The loop has no upper limit, so the method crashes on any list with more than two nodes.
2. 2. Python raises which exception class when code reads or writes an attribute through a None value, such as cur.next when cur is None? Give the class name.

   Type your answer.

3. 3. The code below runs in one program. What does it print, one value per line? ``python x1 = int("256") x2 = int("256") y1 = int("257") y2 = int("257") print(x1 is x2) print(y1 is y2) print(y1 == y2) ``

   True, False, FalseTrue, False, TrueTrue, True, TrueFalse, False, True
4. 4. What is the bug in this function? ``python def insert_after_broken(node, v): node.next = Node(v) ` Called on build(1, 2, 3) with node 1 and the value 9, it prints 1 -> 9 -> None`.

   The method should return the new node, so the caller can keep the list.The new node has no next link, so it must be set to node before the call.The method only works on the last node, so it cannot insert in the middle of a list.The old node.next is overwritten before it is saved, so nodes 2 and 3 are no longer reachable and Python frees them.
5. 5. What is the bug in this function? ``python def remove_broken(head, target): prev = None cur = head while cur is not None and cur.val != target: prev = cur cur = cur.next if cur is None: return head prev.next = cur.next return head ` Removing 4 from 4 -> 5 -> 6 raises AttributeError`.

   When the target is the head, prev is still None, so prev.next = cur.next sets an attribute on None.The loop never finds the value 4, so cur is None when prev.next is set.The loop starts at prev instead of head, so it skips the first node.The loop condition uses and, so it stops before it checks cur.val.