C++ 代码

// 快慢指针
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *entryNodeOfLoop(ListNode *head) {
        if(!head) return nullptr;

        auto first = head, second = head;
        first = first->next, second = second->next->next;  // 方便进入循环
        while(first != second)
        {
            first = first->next;                           // 慢指针一次走一步
            second = second->next->next;                   // 快指针一次走两步

            if(!first || !second) return nullptr;          // 若没有环，则直接返回 nullptr
        }

        // 此时就是第一次的相遇点
        second = head;
        while(first != second)
        {
            first = first->next;
            second = second->next;
        }

        // 此时就是环入口
        return second;
    }
};