Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
题目大意：给定一个链表，判断是否有环？
解题思路：
解法一：快慢指针，如果有环，那么快慢指针总会相遇，有环；否则快的遍历完整个链表，无环。
解法二：HashSet保存节点，如果下一个节点在set中出现过，那么有环，否则直到遍历完整个链表，无环。
Talk is cheap>>
解法一：
public boolean hasCycle(ListNode head) { if (head == null || head.next == null) { return false; } ListNode slow = head; ListNode fast = head; while (fast.next != null && fast.next.next != null) { fast = fast.next.next; slow = slow.next; if (slow == fast) { return true; } } return false; }
解法二：
public boolean hasCycle2(ListNode head) { if (head == null || head.next == null) { return false; } Set<ListNode> set = new HashSet<>(); while (head != null) { if (set.contains(head)) { return true; } set.add(head); head = head.next; } return false; }
原文：http://www.cnblogs.com/aboutblank/p/4446675.html