LeetCode141LinkedListCycle和142LinkedListCycleII

 1 // 不能使用额外的存储空间
 2     public boolean hasCycle(ListNode head) {
 3         // 如果存在环的 两个指针用不一样的速度 会相遇
 4         ListNode fastNode = head;
 5         ListNode slowNode = head;
 6         while (fastNode != null && fastNode.next != null) {
 7             fastNode = fastNode.next.next;
 8             slowNode = slowNode.next;
 9             if (fastNode == slowNode) {
10                 return true;
11             }
12         }
13         return false;
14     }

 1 // 如果有环，相遇的时候与起点相差的步数等于从head到起点的步数
 2     public ListNode detectCycle(ListNode head) {
 3         ListNode pre = head;
 4         ListNode slow = head;
 5         ListNode fast = head;
 6         while (fast != null && fast.next != null) {
 7             fast = fast.next.next;
 8             slow = slow.next;
 9             if (slow == fast) {
10                 while (pre != slow) {
11                     slow = slow.next;
12                     pre = pre.next;
13                 }
14                 return pre;
15             }
16         }
17         return null;
18     }