环形链表的入口节点

https://leetcode.cn/problems/c32eOV/solutions/977000/jian-zhi-offer-ii-022-lian-biao-zhong-hu-8f1m/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    // 环形入口节点，第一次相遇后，将快指针从头节点开始，每次走一步，当
    // 再次与slow指针相遇，则就是入口节点
    ListNode *detectCycle(ListNode *head) {
        if (head == nullptr || head->next == nullptr) return nullptr;
        ListNode* fast = head;
        ListNode* slow = head;

        while(true) {
            if (fast == nullptr || fast->next == nullptr) return nullptr;
            
            slow = slow->next;
            fast = fast->next->next;
            if (fast == slow) break;
        }
        fast = head;
        while(slow != fast)  {
            slow = slow->next;
            fast = fast->next;
        }

        return fast;
    }

  

};