141. Linked List Cycle

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

class Solution 
{
public:
    bool hasCycle(ListNode *head) 
    {
        ListNode *fast=head,*slow=head;
        while(fast!=NULL)
        {
            fast=fast->next;
            if(fast==NULL)
                return false;
            fast=fast->next;
            slow=slow->next;
            if(slow==fast)
                return true;
        }
        return false;
    }
};

慢指针追快指针，追到则必有环。

还有一种更简洁的写法

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
static int wing=[]()
{
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();

class Solution 
{
public:
    bool hasCycle(ListNode *head) 
    {
        ListNode *fast=head,*slow=head;
        while(fast!=NULL&&fast->next!=NULL)
        {
            fast=fast->next->next;
            slow=slow->next;
            if(slow==fast)
                return true;
        }
        return false;
    }
};