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