Given a linked list, return the node where the cycle begins.
If there is no cycle, return null.
Example
Given -21->10->4->5, tail connects to node index 1，return 10
分析
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/** * Definition for ListNode.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int val) {
* this.val = val;
* this.next = null;
* }
* }
*/ public class Solution {
/**
* @param head: The first node of linked list.
* @return: The node where the cycle begins.
* if there is no cycle, return null
*/
public ListNode detectCycle(ListNode head) {
// write your code here
ListNode slow = head, fast = head;
ListNode Pos = null;
while(fast != null && fast.next != null && fast.next.next != null){
slow = slow.next;
fast = fast.next.next;
if(fast == slow){
Pos = slow;
break;
}
}
if(Pos == null) return null;
while(head != Pos){
head = head.next;
Pos = Pos.next;
}
return Pos;
}
} |