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判断链表是否为环（Java版本自己用）

news 来源：原创 2025/4/22 13:58:57

141. 环形链表

核心代码版本：

public class Solution {public boolean hasCycle(ListNode head) {if (head == null) {return false;}ListNode slow = head;ListNode fast = head.next;while (fast != null && fast.next != null) {if (slow == fast) {return true;}//slow = slow.next;fast = fast.next.next;}return false;}
}

深度解析：

下面通过一个例子来解释为什么不能只用

fast != null

来判断链表是否有环而是要加上

 fast.next != null

考虑一个链表：

1 -> 2 -> 3 -> 4 -> null

情况 1：

只判断 fast != null
ListNode fast = head;
while (fast != null) {// 执行访问 fast.next.nextfast = fast.next.next;
}

情况 2：

fast != null && fast.next != null
ListNode fast = head;
while (fast != null && fast.next != null) {// 执行访问 fast.next.nextfast = fast.next.next;
}

ACM版本：

package LC141.CodeFun;import java.util.Scanner;class ListNode {int val;ListNode next;ListNode(int x) {val = x;next = null;}
}public class Main {// 创建基本链表public static ListNode createLinkedList(int[] nums) {if (nums == null || nums.length == 0) {return null;}ListNode head = new ListNode(nums[0]);ListNode current = head;for (int i = 1; i < nums.length; i++) {current.next = new ListNode(nums[i]);current = current.next;}return head;}// 在指定位置创建环public static void createCycle(ListNode head, int pos) {if (head == null || pos < 0) {return;}// 找到尾节点和目标节点ListNode tail = head;ListNode target = head;// 先找到尾节点while (tail.next != null) {tail = tail.next;}// 再找到目标位置的节点for (int i = 0; i < pos && target != null; i++) {target = target.next;}// 将尾节点连接到目标节点，形成环if (target != null) {tail.next = target;}}// 使用快慢指针判断链表是否存在环public static boolean hasCycle(ListNode head) {if (head == null || head.next == null) return false;ListNode slow = head, fast = head;while (fast != null && fast.next != null) {slow = slow.next;fast = fast.next.next;if (slow == fast) return true;}return false;}public static void main(String[] args) {Scanner scanner = new Scanner(System.in);String str = scanner.nextLine();String[] strs = str.split(",");int[] nums = new int[strs.length];for (int i = 0; i < strs.length; i++) {nums[i] = Integer.parseInt(strs[i]);}ListNode listNode = createLinkedList(nums);// 假设第二行输入是环的位置（-1表示无环）int pos = Integer.parseInt(scanner.nextLine());if (pos >= 0) {createCycle(listNode, pos);}boolean result = hasCycle(listNode);System.out.println(result);scanner.close();}
}