题目

https://leetcode-cn.com/problems/linked-list-cycle-ii/

代码

使用java的方式解决

public class Solution {
    public ListNode detectCycle(ListNode head) {
        Set<ListNode>set = new HashSet<ListNode>();

        ListNode node = head;
        while(node!=null){
            if(set.contains(node)){
                return node;
            }else{
                set.add(node);
                node=node.next;
            }
        }
        return null;
    }
}

这种方式需要画图看看，转换其实是一个数学问题。

/**
 * 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){
            return null;
        }
        
        ListNode fast=head;
        ListNode slow=head;
        boolean hasCycle=false;
        while(fast!=null&&fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
            if(fast==slow){
                hasCycle=true;
                break;
            }
        }
        
        if(hasCycle){
            ListNode pp = head;
            while(pp!=slow){
                pp=pp.next;
                slow=slow.next;
            }
            return pp;
        }else{
            return null;
        }
    }
}