• 原题连接：141. 环形链表

1- 思路

快慢指针+推导

• ① 利用快慢指针，定位环
• ② 根据环，从头出发一个指针，从环处出发一个指针
  • 两者相遇的地方就是环的入口

2- 实现

⭐142. 环形链表 II——题解思路

public class Solution {
    public ListNode detectCycle(ListNode head) {
        if(head==null){
            return head;
        }
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next!=null && fast.next.next!=null){
            slow = slow.next;
            fast = fast.next.next;
            if(slow==fast){
                ListNode index1 = head;
                ListNode index2 = slow;
                while(index1!=index2){
                    index1 = index1.next;
                    index2 = index2.next;
                }
                return index1;
            }
        }
        return null;
    }   
}

3- ACM 实现

public class hashCycle {

    public static class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
            next = null;
        }
    }


    public static ListNode detectCycle(ListNode head) {
        if(head==null){
            return head;
        }
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next!=null && fast.next.next!=null){
            slow = slow.next;
            fast = fast.next.next;
            if(slow==fast){
                ListNode index1 = head;
                ListNode index2 = slow;
                while(index1!=index2){
                    index1 = index1.next;
                    index2 = index2.next;
                }
                return index1;
            }
        }
        return null;
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("输入链表长度");
        int n = sc.nextInt();
        ListNode head = null,tail=null;
        for(int i = 0 ; i < n;i++){
            ListNode nowNode  = new ListNode(sc.nextInt());
            if(head==null){
                head = nowNode;
                tail = nowNode;
            }else{
                tail.next = nowNode;
                tail = nowNode;
            }
        }
        System.out.println("输入环的位置");
        int index = sc.nextInt();
        ListNode cycleNode = head;
        while (index>0){
            cycleNode = cycleNode.next;
            index--;
        }
        tail.next = cycleNode;
        System.out.println("结果是"+detectCycle(head).val);
    }
}