LeetCode 142 Linked List Cycle II

 

要点： 

1.  出了第一个while loop后，要先判断fast指针的情况。

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

        return head;
    }
}