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

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

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

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/**
 * 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) {
        ListNode cycleStart=null,
                singleStepNode=null,
                doubleStepNode=null;
        boolean isCycle=false;
        
        if(head==null)
            return cycleStart;
        
        singleStepNode =head;
        doubleStepNode=head;
        
        while(singleStepNode!=null && doubleStepNode!=null && doubleStepNode.next!=null)
        {
            singleStepNode=singleStepNode.next;

            doubleStepNode=doubleStepNode.next.next;
            
            if(singleStepNode==doubleStepNode)
            {
                isCycle=true;
                break;
            }
        }
        
        if(isCycle)
        {
            singleStepNode=head;
            while(singleStepNode != doubleStepNode)
            {
                singleStepNode=singleStepNode.next;
                doubleStepNode=doubleStepNode.next;
            }
            
            cycleStart=singleStepNode;
        }
        
        return cycleStart;
    }
}

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