题目：

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代码(首刷看解析 2024年1月13日）：

class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if (head == nullptr) return nullptr;
        ListNode* fast = head;
        ListNode* slow = head;
        while (true) {
            if(fast->next == nullptr || fast->next->next == nullptr) return nullptr;
            fast = fast->next->next;
            slow = slow->next;
            if(fast == slow) break;
        }
        fast = head;
        while (fast != slow) {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};

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