Two singly linked lists intersect by reference if and only if which condition holds?

• A. The tails of the two lists are the same node object
• B. The heads of the two lists are the same node object
• C. The lists have equal lengths
• D. The first nodes have equal values

Answer: (A)

Intersections by reference mean that the two lists share actual nodes from some point onward. From that merge point to the end, both lists are the same sequence of node objects, so their tails are the same node object. That tail equality is the precise signal that the lists intersect by reference: if the tails are the same, there is a shared tail; if the tails are different, there cannot be a shared tail.

If the heads were the same node, that would be a sufficient condition for intersection, but it’s not necessary—the lists could join later and still intersect. List lengths don’t determine intersection, since two lists can have equal or different lengths and still either intersect or not. The values at the first nodes don’t indicate reference intersection, because intersection depends on node identity, not the value stored in nodes.