题解 | 判断链表中是否有环

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

        ListNode* slow = head;       // 慢指针，每次走一步
        ListNode* fast = head->next; // 快指针，每次走两步

        while (slow != fast) {
            // 如果快指针到达链表末尾，说明没有环
            if (fast == nullptr || fast->next == nullptr) {
                return false;
            }

            slow = slow->next;         // 慢指针走一步
            fast = fast->next->next;   // 快指针走两步
        }

        // 快慢指针相遇，说明有环
        return true;
    }
};