成员访问类型为'struct ListNode'的空指针

Question

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* walker = head;
        ListNode* runner = head;
        while(runner->next != NULL && walker->next != NULL){
            walker = walker->next;
            runner = runner->next->next;
            if(walker == runner) return true;
        }
        return false;
    }
};

我正在练习一个看起来很简单的面试代码.我必须返回一个bool来确定单链表是否有循环.我做了两个指针步行者,它移动了1步和每次迭代移动2步的跑步者.

但是这段代码给了我一个错误:

Line 15: member access within null pointer of type 'struct ListNode'

是什么原因导致错误？

Answer 1

您只能确保它runner->next不是null,但是在分配之后

runner = runner->next->next;

runner 可以变为空.