Linked List Cycle

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

Follow up:

Can you solve it without using extra space?

定义两个指针，一个快，一个慢，快的追到慢的则存在环。

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