LeetCode141. 环形链表&&142. 环形链表 II

141. 环形链表

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

142. 环形链表 II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(head == nullptr || head->next == nullptr) return nullptr;
        ListNode *fast = head;
        ListNode *slow = head;

        while(fast && fast->next) {
            fast = fast->next->next;
            slow = slow->next;
            if(fast == slow) {
                ListNode *temp = head;
                while(fast != temp) {
                    fast = fast->next;
                    temp = temp->next;
                }
                return temp;
            }
        }
        return nullptr;
    }
};