141

141.环形链表

所谓环形就是最后元素是否相等

即追赶者，与逃跑者的关系；

使用快慢指针

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

bool hasCycle(struct ListNode *head) {
    if (head == NULL || head->next == NULL) {
        return false;
    }    
    struct ListNode *p = head;
    struct ListNode *q = head->next;
    while (p != q) {
        if (q == NULL || q->next == NULL) {
            return false;
        }
        p = p->next;
        q = q->next->next;
    }
    return true;
}

优化思维

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

bool hasCycle(struct ListNode *head) {
    struct ListNode *p = head, *q = head;
    if (p == NULL) return false;
    do {
        p = p->next;
        q = q->next;
        if (q == NULL || q->next == NULL) return false;
        q = q->next;
    } while (p != q);
    return true;
}