Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
Constraints:
-
The number of the nodes in the list is in the range [0, 10?].
-
-10? <= Node.val <= 10?
-
pos is -1 or a valid index in the linked-list.
Follow up: Can you solve it using O(1) (i.e. constant) memory?
方法一：我写的方法
两指针p1、p2，每次循环中，p1走1步，p2走1步，再走1步，其间判断p1、p2是否相等。若相等，则链有环。若p2为null，则没有。
方法二：别人写的方法
-
Use two pointers, walker and runner.
-
walker moves step by step. runner moves two steps at time.
-
if the Linked List has a cycle walker and runner will meet at some point.
import com.lun.util.SinglyLinkedList.ListNode;
public class LinkedListCycle {
// 方法一：我写的方法
public boolean hasCycle1(ListNode head) {
if (head == null)
return false;
ListNode p1 = head, p2 = head.next;
while (true) {
if (p2 == null)
break;
if (p1 == p2) {
return true;
} else {
p1 = p1.next;
p2 = p2.next;
if (p2 == null) {
break;
} else {
if (p1 == p2)
return true;
p2 = p2.next;
}
}
}
return false;
}
// 方法二：别人写的方法
public boolean hasCycle2(ListNode head) {
ListNode walker = head;
ListNode runner = head;
while(runner != null && runner.next != null) {
walker = walker.next;
runner = runner.next.next;
if(walker == runner) return true;
}
return false;
}
}
import static org.junit.Assert.*;
import org.junit.Test;
import com.lun.util.SinglyLinkedList.ListNode;
public class LinkedListCycleTest {
@Test
public void test1() {
LinkedListCycle obj = new LinkedListCycle();
ListNode node1 = new ListNode(3), node2 = new ListNode(2),
node3 = new ListNode(0), node4 = new ListNode(-4);
node1.next = node2;
node2.next = node3;
node3.next = node4;
node4.next = node2;
assertTrue(obj.hasCycle1(node1));
assertTrue(obj.hasCycle2(node1));
}
@Test
public void test2() {
LinkedListCycle obj = new LinkedListCycle();
ListNode node1 = new ListNode(1), node2 = new ListNode(2);
node1.next = node2;
node2.next = node1;
assertTrue(obj.hasCycle1(node1));
assertTrue(obj.hasCycle2(node1));
}
