Define the intersection of two singly linked lists and describe a standard approach to detect it.

• A. Two lists intersect if they start at the same head node.
• B. They intersect if their tail values are equal.
• C. They share a common node by reference; detect by aligning lengths and moving together until a common node is found.
• D. Intersection can be detected by comparing list lengths only.

Answer: (C)

Two singly linked lists intersect when they share a node by reference; from that node onward, both lists are the same sequence of nodes in memory. It’s about the actual node objects, not just values—two lists can look alike in value but not be the same underlying nodes, so sharing a tail node is what defines an intersection.

A standard way to detect this is to align the lists by length and then traverse them together. First, determine the length of each list. If one is longer, advance its head pointer by the difference in lengths so both pointers have the same number of nodes to traverse until the end. Then move both pointers forward in lockstep, checking for pointer equality. The first time the two pointers reference the same node, that node is the intersection. If you reach the end of both lists without a common reference, there is no intersection.

This approach runs in linear time, O(m + n), and uses constant extra space, O(1). It’s correct precisely because once aligned, any shared tail is encountered at the same moment by both pointers, guaranteeing detection if an intersection exists.