题解 | #判断链表中是否有环#

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

/**
 * 
 * @param head ListNode类 
 * @return bool布尔型
 */
bool hasCycle(struct ListNode* head ) {
    // write code here
    if(NULL == head)
        return false;
    //p1快指针，p2慢指针
    struct ListNode* p1 = head;
    struct ListNode* p2 = head;
    for(;;)
    {
        if(NULL == p1 || NULL == p1->next || NULL == p1->next->next)
            return false;
        p2 = p2->next;
        p1 = p1->next->next;
        if(p1 == p2)
            return true;
    }
}