LeetCode 每日一题 141. 环形链表
141. 环形链表
输入：head = [3,2,0,-4], pos = 1
输出：true
解释：链表中有一个环，其尾部连接到第二个节点。
题目描述：给你一个链表的头节点 head ，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，则链表中存在环。 为了表示给定链表中的环，评测系统内部使用整数 pos 来表示链表尾连接到链表中的位置（索引从 0 开始）。注意：pos 不作为参数进行传递 。仅仅是为了标识链表的实际情况。
如果链表中存在环 ，则返回 true 。 否则，返回 false 。
思路：快慢指针
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。
代码：
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
ListNode *l1 = head, *l2 = head;
while(l2 != NULL && l2->next != NULL)
{
l1 = l1->next;
l2 = l2->next->next;
if (l1 == l2) return true;
}
return false;
}
};
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