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leetcode:环形链表

王小沫 2022-03-26 阅读 57
算法java

力扣141:

给定一个链表,判断链表中是否有环。

如果链表中有某个节点,可以通过连续跟踪 next 指针再次到达该节点,则链表中存在环

如果链表中存在环,则返回 true 。 否则,返回 false

package com.lz;

import java.util.HashSet;
import java.util.Set;

public class LinkCycle {

	public static class ListNode{ 
		int val;
		ListNode next; 
		
		public ListNode(int val, ListNode next) { 
			this.val = val; 
			this.next = next; 
		}
	}
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ListNode node5,node4,node3,node2,node1;
		node5 = new ListNode(5,null); 
		node4 = new ListNode(4,node5); 
		node3 = new ListNode(3,node4);
		node2 = new ListNode(2,node3); 
		node1 = new ListNode(1,node2); 
		node5.next=node3;
		System.out.println(hasCycle(node1)); 
		System.out.println(hasCycle2(node1)); 
	}
	//迭代
	public static boolean hasCycle(ListNode head) {
		Set<ListNode> set=new HashSet<>();
		while(head!=null) {
			if(!set.add(head)) {
				return true;
			}
			head=head.next;
		}
		return false;
	}
	//双指针
	public static boolean hasCycle2(ListNode head) {
		if(head==null||head.next==null) {
			return false;
		}
		ListNode slow=head;
		ListNode quick=head.next;
		while(slow!=quick) {
			if(quick==null||quick.next==null) {
				return false;
			}
			slow=slow.next;
			quick=quick.next.next;
		}
		return true;
	}
	
}

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