leetcode linked-list-cycle-ii(Java)

leetcode题目

 linked-list-cycle-ii

题目描述

 * 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?

思路

 * 1、找到链表中的环中的一个节点(快慢指针相遇处)
 * 2、一个指针从相遇处出发，一个指针从链表头节点出发，相遇处即为链表中环的起始点
   (也可利用环中节点遍历一圈，确定环的长度N，然后利用两个指针一个先走N步，之后另一指针再走，
   相遇节点即为环的起始点)

代码

package com.leetcode.list;

/**
 * 题目：
 * linked-list-cycle-ii
 * 
 * 题目描述:
 * 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?
 */
public class ListCircleBegin {

	static class ListNode{
		 int val;
		 ListNode next;
		 ListNode(int x) {
		     val = x;
		     next = null;
		 }
	}
	
	/**
	 * 思路：
	 * 1、找到链表中的环中的一个节点(快慢指针相遇处)
	 * 2、一个指针从相遇处出发，一个指针从链表头节点出发，相遇处即为链表中环的起始点
	 * 
	 * @param head 头节点
	 */
    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) {
        	return null;
        }
        // 找到环中的一个节点
        ListNode nodeInCycle = findOneNodeInCircle(head);
        if (nodeInCycle == null) {
        	return null;
        }
        // 两个指针，一个指向头节点，一个指向环中的这个节点
        ListNode first = nodeInCycle;
        ListNode second = head;
        while (first != null && second != null) {
        	// 两指针相遇，找到环的起始节点
        	if (first == second) {
        		return first;
        	}
        	first = first.next;
        	second = second.next;
        }
        return null;
    }
    
    private ListNode findOneNodeInCircle(ListNode head) {
    	ListNode fast = head;
    	ListNode slow = head;
    	while (fast.next != null && fast.next.next != null) {
    		fast = fast.next.next;
    		slow = slow.next;
    		// 快慢指针相遇
    		if (fast == slow) {
    			return fast;
    		}
    	}
    	return null;
    }
    
	public static void main(String[] args) {
		ListNode head = createTestLinkedList();
		ListNode newHead = new ListNode(13);
		newHead.next = head;
		head.next.next = newHead;
		ListNode begin = new ListCircleBegin().detectCycle(newHead);
		System.out.println(begin.val);
	}
	
    private static ListNode createTestLinkedList() {
    	ListNode head = new ListNode(0);
    	ListNode curNode = head;
        for (int i = 1; i < 10; i++) {
            curNode.next = new ListNode(i);
            curNode = curNode.next;
        }
        return head;
    }

}