LeetCode 141 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?

使用两个指针，一个指针每次直走一步，另一个指针每次走两步，如果是链表有环，那么两个指针总会相遇，否则快指针访问到链表尾部就结束。

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