Linked List Cycle II

Posted 2022-12-01 大数据最好

问题描述：

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Note: Do not modify the linked list.

Follow up:
Can you solve it without using extra space?

代码实现：

/**
 * 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) return null;
        ListNode fast=head;
        ListNode slow=head;
        do
            if(fast!=null) fast=fast.next;
            if(fast!=null)
            	fast=fast.next;
            else
                return null;
            
            slow=slow.next;
        while(fast != slow);   
        
        slow=head;
        while(fast!=slow)
            fast=fast.next;
            slow=slow.next;
        

        return slow;