everystep

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* slow = head;
        ListNode* fast = head;

        // 检测环
        while (true) {
            if (fast == nullptr || fast->next == nullptr) return nullptr;
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) break; // 当快慢指针相遇，退出循环
        }

        // 寻找环的入口
        slow = head;
        while (slow != fast) {
            slow = slow->next;
            fast = fast->next;
        }
        return slow; // 返回环的入口节点
    }
};