(链表 双指针) leetcode 141. Linked List Cycle

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.

Follow up:

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

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用中文来说，这个题就是判断一个链表是否存在环。可以用双指针来解决。即可以建立一个慢指针和快指针，最后两个指针相遇，就可以判断相等。

C++代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head) {
    struct ListNode *slow,*fast;
    slow = head;
    fast = head;
    while(fast && fast->next){  //如果fast=NULL或fast->next=NULL就说明了链表一定没有环，只是一个单链表而已。
        slow = slow->next;
        fast = fast->next->next;
        if(slow == fast)
            return true;
    }
    return false;
}