Leetcode-141-判断链表有没有环

Leecode-141 Linked List Cycle

思路：快慢指针

题目描述:

判断链表有没有环

Solution：

• 使用双指针，一个指针每次移动一个节点，一个指针每次移动两个节点，如果存在环，那么这两个指针一定会相遇。

Java

Solution :

/**
 * 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 l1 = head,l2 = head.next;
        while(l1 != null & l2 !=null && l2.next != null){
            if (l1 == l2) return true;
            l1 = l1.next;
            l2 = l2.next.next;
        }
        return false;
    }
}

Python

Solution :

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if not head: return False
        l1 = head
        l2 = head.next
        while l1 and l2 and l2.next:
            if l1 == l2:
                return True
            l1 = l1.next
            l2 = l2.next.next
        return False

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