Linked List Cycle II

LeetCode Q 142 - Linked List Cycle II

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.

Solution

Code:

public ListNode detectCycle(ListNode head) {
	if (head == null) return null;
	
	ListNode slow = head, fast = head;
	while (fast != null && fast.next != null) {
		slow = slow.next;
		fast = fast.next.next;
		if (slow == fast)
			break;
	}

	if (fast == null || fast.next == null)
		return null;
	
	while (head != slow) {
	    head = head.next;
    	slow = slow.next;
	}

	return slow;
}