Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
Note: Do not modify the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.
Follow-up:
Can you solve it without using extra space?
public class Solution {
public ListNode checkLoop(ListNode head) {
ListNode slow = head, fast = head;
while (slow != null && fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast)
return fast;
}
return null;
}
public ListNode detectCycle(ListNode head) {
if (head == null || head.next == null)
return null;
ListNode cycle = checkLoop(head);
if (cycle == null)
return cycle;
ListNode temp = head;
while (cycle != temp) {
cycle = cycle.next;
temp = temp.next;
}
return cycle;
}
}
