LeetCode 环形链表

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本文介绍了一种使用双指针法检测链表中是否存在环的有效算法。通过一个快指针和一个慢指针，快指针每次移动两步，慢指针每次移动一步，若存在环则快指针必定追上慢指针。

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环形链表

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.

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.

Example 3:

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

思路

双指针法:一个快指针，一个慢指针，快指针每次移动2步，慢指针每次移动1步，如果没有环，那快指针会先到达尾部结束，如果有环，快指针会追上慢指针。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head) {
    if(head == NULL || head->next == NULL)
        return false;
    struct ListNode *slow = head;
    struct ListNode *fast = head;
    while(slow && fast && fast->next)
    {
        
        slow = slow->next;
        fast = fast->next->next;
        if(slow == fast) return true;
    }
    return false;
}

时间复杂度为O(n),空间复杂度为O(1)