Linked List Cycle Problems

The Linked List Cycle problem is:

Given a linked list, determine if it has a cycle in it. Can you solve it without using extra space?

A follow up problem is:

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.Can you solve it without using extra space?

The first problem can be solved by the Floyd’s Cycle-Finding Algorithm:

Traverse linked list using two pointers: move one pointer by one and other pointer by two. If these pointers meet at some node then there is a loop. If pointers do not meet then linked list doesn’t have loop.

As for the second problem, it is also based on the Floyd’s Cycle-Finding Algorithm. However, after detecting loop, we need to find the starting point of the cycle. The most convinient way is to use two pointers, one points to the head node, and the other points to the node that pointers meet when detecting loop. Then move both the two pointers by one respectively in each iteration, until they meet. The node they meet is the starting point of the cycle.

- See more at:http://bo-yang.github.io/2014/07/13/linked-list-cycle/#sthash.I1lYsJyS.dpuf

http://bo-yang.github.io/2014/07/13/linked-list-cycle