linked list cycle ii.java

/**
 * 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 p1 = head;
        ListNode p2 = head;
        boolean isCycle = false;
        
        while (p2.next != null && p2.next.next != null) {
            p1 = p1.next;
            p2 = p2.next.next;
            if (p1.equals(p2)) {
                isCycle = true;
                break;
            }
        }
        
        if (!isCycle) return null;
        p1 = head;
        
        while (!p1.equals(p2)) {
            p1 = p1.next;
            p2 = p2.next;
        }
        
        return p1;
    }
}