环形链表

整理思路：这个解法还是很普遍的，就是挨个遍历。直到快的追上慢的。但是有个问题如下：

 while(fast.next!=null&&fast.next.next!=null){    //？调换两个条件的位置，结果不一样        
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow)return true;
        }

正确代码：

public class num_18 {
	public static class ListNode {
	      int val;
	      ListNode next;
	      ListNode(int x) { val = x; }
	  }

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ListNode node1 = new ListNode(1);
		ListNode node2 = new ListNode(2);
		ListNode node3 = new ListNode(2);
		ListNode node4 = new ListNode(1);
		ListNode node5 = new ListNode(4);
		ListNode node6 = new ListNode(5);
		ListNode node7 = new ListNode(1);
		ListNode node8 = new ListNode(9);
		node1.next = node2;
		node2.next = node3;
		node3.next = node4;
		node4.next = node5;
		node5.next = node6;
		node6.next = node7;
		node7.next = node8;
		System.out.println(hasCycle(node1));
	}
	
	public static boolean hasCycle(ListNode head) {
        if(head==null||head.next==null)
        	return false;
        ListNode fast = head,slow = head;
        while(fast.next!=null&&fast.next.next!=null) {
        	fast = fast.next.next;
        	slow = slow.next;
        	if(fast==slow)
        		return true;
        }
        return false;
    }
}