Explain how to find the middle node of a singly linked list in one pass.

• A. Slow pointer moves one step; fast pointer moves two steps; when fast reaches end, slow is middle.
• B. Move both pointers one step at a time; the node where they meet is the middle.
• C. Use a stack; when stack size equals half of length.
• D. The middle node is the first node.

Answer: (A)

Two pointers moving at different speeds, a slow pointer and a fast pointer, is the key idea. Start both at the head, then in each step move the slow pointer one node forward and the fast pointer two nodes forward. Because the fast pointer travels twice as fast, it reaches the end of the list in about half as many steps as the slow pointer. When the fast pointer arrives at the end (null), the slow pointer has advanced through roughly half the list, so it sits at the middle node. In an odd-length list, this is exactly the middle; in an even-length list, it ends up on the second of the two middle nodes, which is a common convention. This approach runs in O(n) time and O(1) extra space, and achieves the goal in a single pass.