letecode [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.

题目大意：

 　　判断一个链表中是否存在环。

理  解：

 　　设置“快”“慢”指针。“快”指针每次后移两个，“慢”指针每次后移一个，当“快”指针碰到“慢”指针，则说明存在环；若“快”指针指向空，说明不存在环。

代 码 C++：

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

运行结果：

 　　执行用时 : 24 ms, 在Linked List Cycle的C++提交中击败了45.37% 的用户

　　内存消耗 : 9.9 MB, 在Linked List Cycle的C++提交中击败了11.19% 的用户