*Linked List Cycle

题目：

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

Analysis

If we have 2 pointers - fast and slow. It is guaranteed that the fast one will meet the slow one if there exists a circle.

The problem can be demonstrated in the following diagram:

    public boolean hasCycle(ListNode head) {
        ListNode fast = head;
        ListNode slow = head;
        
        while(fast!=null&&fast.next!=null&&fast.next.next!=null)
        {
            fast = fast.next.next;
            slow = slow.next;
            
            if(fast==slow)return true;
            
        }
        
        return false;
        
    }

reference：http://www.programcreek.com/2012/12/leetcode-linked-list-cycle/