Given a linked list, determine if it has a cycle in it.
Follow up:
Can you solve it without using extra space?
Tips:判斷一個連結串列中是否有環。
思路：設定兩個指標，一個快（fast）一個慢（slow），fast指標每次走兩步，slow每次走一步。當兩個指標相遇，即存在環。若連結串列中沒有環，則fast指標先到達null；
public boolean hasCycle(ListNode head) { if (head == null || head.next == null) return false; boolean hasCy = false; ListNode fast = head; ListNode slow = head; while (fast!=null) { if(fast.next!=null){ fast=fast.next; if(fast.next!=null){ fast=fast.next; slow=slow.next; if (fast == slow) { hasCy = true; break; } }else return false; }else return false; } return hasCy; }