自己做
解：三指针
java
复制代码
/**
* 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) //必定无环
return null;
ListNode p = head;
ListNode q = head.next;
ListNode r = head.next;
while(r != null){
r = r.next; //快指针
if(r != null)
r = r.next;
q = q.next; //慢指针
if(q == p) //遇到入口
return p;
if(r == q) //快指针追上了慢指针（转了一圈）
p = p.next;
}
//无环
return null;
}
}
看题解
java
复制代码
public class Solution {
public ListNode detectCycle(ListNode head) {
if (head == null) {
return null;
}
ListNode slow = head, fast = head;
while (fast != null) {
slow = slow.next;
if (fast.next != null) {
fast = fast.next.next;
} else {
return null;
}
if (fast == slow) {
ListNode ptr = head;
while (ptr != slow) {
ptr = ptr.next;
slow = slow.next;
}
return ptr;
}
}
return null;
}
}
作者：力扣官方题解
链接：https://leetcode.cn/problems/linked-list-cycle-ii/solutions/441131/huan-xing-lian-biao-ii-by-leetcode-solution/
来源：力扣（LeetCode）
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