141. Linked List Cycle

快慢指针O(n)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        auto slow = head, fast = head;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) return true; // 因为slow和fast都是从head开始，所以不能把这个判断条件放在最开始
        }
        return false;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (!head || !head->next) return false;
        auto slow = head, fast = head;
        do {
            slow = slow->next;
            fast = fast->next->next;
        } while (fast && fast->next && slow != fast);
        return slow == fast;
    }
};

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (!head || !head->next) return false;
        auto slow = head, fast = head->next;
        while (fast && fast->next && slow != fast) {
            slow = slow->next;
            fast = fast->next->next;
        }
        return slow == fast;
    }
};