LinkedListCycleII.java

package leetcode;

/**
 * @author eko
 *
 *
 * 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 LinkedListCycleII {
    public static void main(String[] args) {

    }

    public ListNode detectCycle(ListNode head) {
        if (head == null) return null;
        ListNode walker = head;
        ListNode runner = head;
        while (runner.next != null && runner.next.next != null) {
            walker = walker.next;
            runner = runner.next.next;
            if (walker == runner) break;
        }
        if (runner.next != null && runner.next.next != null) {
            while (head != walker) {
                ListNode newHead = walker.next;
                while (newHead != walker) {
                    if (newHead == head) return newHead;
                    newHead = newHead.next;
                }
                head = head.next;
            }
            return head;
        } else {
            return null;
        }
    }

    public static class ListNode {
        int val;
        ListNode next;

        ListNode(int x) {
            val = x;
            next = null;
        }
    }
}