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?
Craking interview書上原題，快慢指針，話題較簡單說明。
/**
* 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) {
// IMPORTANT: Please reset any member data you declared, as
// the same Solution instance will be reused for each test case.
ListNode slow = head;
ListNode fast = head;
while(fast != null && fast.next != null){
slow = slow.next;
fast = fast.next.next;
if(slow == fast) break;
}
if(fast == null || fast.next == null) {
return null;
}
slow = head;
while(slow != fast){
slow = slow.next;
fast = fast.next;
}
return slow;
}
}
