LeetCode Linked List Cycle

/**
 * 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 == NULL) return false;
        ListNode* fast = head;
        ListNode* slow = head;
        while (step(fast, 2) && step(slow, 1)) {
            if (fast == slow) break;
        }
        return fast != NULL;
    }
    
    bool step(ListNode* &node, int n) {
        while (node != NULL && n > 0) {
            node = node->next;
            n--;
        }
        return n == 0;
    }
};

水一发

第二轮：

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

必做题

/**
 * 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 == NULL) {
            return false;
        }
        ListNode* fast = head->next;
        ListNode* slow = head;
        while (fast != NULL && fast->next != NULL) {
            if (fast == slow) {
                break;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        return fast == slow;
    }
};