LeetCode141:Given a linked list, determine if it has a cycle in it.

LeetCode141:Given a linked list, determine if it has a cycle in it.

给定链表，判断是否存在环

1、该题的思路，如果使用额外空间，解决就很容易。我们用map存储每个节点引用，如果该引用的值大于1，就说明在该位置存在环。

public boolean hasCycle(ListNode head)
	{
		if (head == null)
			return true;
		if (head.next == null && head.next == head)
			return false;
		Map<ListNode, Integer> map = new HashMap<ListNode, Integer>();
		ListNode p = head;
		while (p != null)
		{
			if (map.containsKey(p)&&map.get(p) == 1)
				return true;
			map.put(p, 1);
			p = p.next;
		}
		return false;
	}

同样可以用该方法返回环存在的位置

public ListNode detectCycle(ListNode head)
	{
		if (head == null)
			return null;
		if (head.next == null && head.next == head)
			return head;
		Map<ListNode, Integer> map = new HashMap<ListNode, Integer>();
		ListNode p = head;
		while (p != null)
		{
			if (map.containsKey(p) && map.get(p) == 1)
				return p;
			map.put(p, 1);
			p = p.next;
		}
		return null;
	}