leetcode--Linked List Cycle II

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?

 

/**
 * 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 entry = null;
		boolean flag = true;
		if(head != null){
			ListNode speedOne = head;
			ListNode speedTwo = head;
			if(head.next != null){
				speedOne = speedOne.next;
				if(speedOne.next != null)
					speedTwo = speedOne.next;
				else
					flag = false;
			}
			else
				flag = false;
			while(flag){
				if(speedOne == speedTwo)
					break;
				else{
					speedOne = speedOne.next;
					if(speedTwo != null && speedTwo.next != null){
						speedTwo = speedTwo.next;
						speedTwo = speedTwo.next;
					}
					else{
						flag = false;
						break;
					}
				}
			}
			if(flag){
				speedOne =  head;
				while(speedOne != speedTwo){
					speedOne = speedOne.next;
					speedTwo = speedTwo.next;
				}
				entry = speedOne;
			}
			else
				entry = null;
		   
	    }
	    return entry;
    }
}

　　

 rewrite the code as the following:

public class Solution {
    public ListNode detectCycle(ListNode head) {
       //find the intersection ListNode of speed one and speed two
		ListNode intersectNode = null;
		if(head != null && head.next != null){
			ListNode speedOne = head;
			ListNode speedTwo = head.next;
			while(speedTwo != null){
				if(speedOne == speedTwo)
					break;
				
				speedOne = speedOne.next;
				speedTwo = speedTwo.next;
				if(speedTwo != null)
					speedTwo = speedTwo.next;
			}
		
			//Once speedTwo meets speedOne, we reset speedOne to head and change 
			//the speed of speedTwo to one
			if(speedTwo != null){
				speedOne = head;
				speedTwo = speedTwo.next;
				while(speedOne != speedTwo){
					speedOne = speedOne.next;
					speedTwo = speedTwo.next;
				}
				intersectNode = speedOne;
			}
		}
		return intersectNode;
    }
}