[每日 LeetCode] 141. Linked List Cycle

Description:

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?

思路：本题要求判断链表中是否存在环。考虑使用两个指针，一个走的快，一个走的慢，如果存在环，则两者必然相遇，返回 true。

C++ 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) {
        if(head == NULL)
    		return false;
    	ListNode *fast = head;
    	ListNode *slow = head;
    	while(fast->next && fast->next->next)
    	{
    		fast = fast->next->next;
    		slow = slow->next;
    		if(fast == slow)
    			return true;
    	}
    	return false;
    }
};

运行时间：12ms

运行内存：9.7M