141. Linked List Cycle - jiejackyzhang/leetcode-note GitHub Wiki

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

Follow up: Can you solve it without using extra space?

Linked List类题目。

可设置两个指针，一快一慢，若快指针追上了慢指针，则说明有cycle。

/**
 * 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) return false;
        ListNode slow = head;
        ListNode fast = head;
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if(fast == slow || fast.next == slow) return true;
        }
        return false;
    }
}