[LeetCode 141] Linked List Cycle (easy)

Given a linked list, determine if it has a cycle in it.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

image

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

image

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

image

Follow up:

Can you solve it using O(1) (i.e. constant) memory?

Solution

使用快慢指针的解法。从链表的头部开始，快指针每次走两步，慢指针每次走一步。

• 有环： 快慢指针一定会相遇 （证明过程可以再看看）
• 没有环： 如果循环完一遍都么有相遇，就是没有环

Code

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if (head == null || head.next == null)
        {
            return false;
        }
        
        ListNode fast = head.next.next;
        ListNode slow = head.next;
        
        while (fast != null && fast.next != null)
        {
            if (fast == slow)
            {
                return true;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        
        return false;
    }
}