1. Without using extra space means the space is O(1)
Analysis:
Set a fast runner and a slow runner. The fast runner jumps 2 nodes at one time, and the slow runner jumps 1 nodes at one time. At the beginning, they all at the head node.
If there is a cycle in the linked list, the fast runner will catch up and surpass the slow runner several times.
Let's say the linked list has N nodes, K independent nodes and a cycle of N-K nodes.
The distance each runner has are Dfast and Dslow, and they meet for T times.
so, Dfast = 2*Dslow
Dfast - Dslow = (N-K)*T
so, Dslow = (N-K)*T