142_LinkedListCycleII.java

/*
 * Given a linked list, return the node where the cycle begins. If there is no
 * cycle, return null.
 * Follow up:
 * Can you solve it without using extra space?
 */
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if(head == null) return null;
        ListNode slow = head;
        ListNode fast = head;
        
        while(fast.next != null) {
            fast = fast.next.next;
            if(fast == null) return null;
            slow = slow.next;
            if(fast == slow) {
                slow = head;
                while(slow != fast) {
                    slow = slow.next;
                    fast = fast.next;
                }
                return fast;
            }
        }
        return null;
    }
}