题解 | #判断链表中是否有环#

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
#include <cstddef>
#include <exception>
#include <stdexcept>
class Solution {
  public:
    bool hasCycle(ListNode* head) {
        ListNode  *fast = head;   //利用两个指针，一个走得快，一个走的慢，如果有环路那么快的一定会追上慢的
        ListNode  *slow = head;
        if (head == NULL || head->next == NULL)
            return false;
        while (fast!= NULL) {
            if (fast->next==NULL) {
                return false;
            }
            fast = fast->next->next;
            slow = slow->next; 
            if (slow == fast) {
                return true;
            }
        }
        return false;
    }
};