Which XXX should replace the missing statement in the following BSTReplaceChild algorithm?

BSTReplaceChild(parent, currentChild, newChild) {
if (parent⇢left != currentChild && parent⇢right != currentChild)
return false

if (parent⇢left == currentChild)
parent⇢left = newChild
else
parent⇢right = newChild

XXX
newChild⇢parent = parent
return true
}

Question

Which XXX should replace the missing statement in the following BSTReplaceChild algorithm?

BSTReplaceChild(parent, currentChild, newChild) {
   if (parent⇢left != currentChild && parent⇢right != currentChild)
      return false

if (parent⇢left == currentChild)
      parent⇢left = newChild
   else
      parent⇢right = newChild

XXX
      newChild⇢parent = parent
   return true
}

BlackTom AI · Accepted Answer

In the BSTReplaceChild routine, after updating the parent's child pointer to point to newChild, the next step is to establish the back-reference from the newChild to its parent.

Option: if (newChild != null)
   newChild⇢parent = parent
   return true

Why this makes sense: If newChild is a valid node, it is essential to set its parent field to the current parent so that traversals or future operations that rely on parent pointers remain correct. The guard newChild != null prevents attempting to dereference a null pointer, which would cause a runtime error.

Why the guard is necessary: If newChild were null (for example, when removing a node or replacing with an empty placeholder), there is no node to link to a parent, so skipping the assignment avoids a null dereference and preserves the invariant that a non-existent child has no parent.

Why this would be incorrect if omitted or placed incorrectly: If you always assign newChild⇢parent = parent without checking for null, you risk a null dereference when newChild is null, leading to undefined behavior or a crash. Conversely, if you place the assignment in a block that executes regardless of newChild's nullity, you could attempt to access properties of null, which is invalid.

In summary, guarding the assignment with if (newChild != null) ensures correctness for both the non-null and null scenarios, attaching the parent only when a real child exists.

类似问题

How many children can a node have in a binary tree?

A perfect binary tree has leaves at least a half of the total number of nodes.

How many nodes in a perfect binary tree if the tree has 4 levels?

Which XXX would replace the missing statement in the given RBTreeReplaceChild utility function? RBTreeReplaceChild(parent, currentChild, newChild) { if (parent⇢left == currentChild) return RBTreeSetChild(parent, "left", newChild) XXX return RBTreeSetChild(parent, "right", newChild) return false }

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