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141. Linked List Cycle

题目

 Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space? 

解答

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

        while (fast!=NULL&&fast->next!=NULL)
        {
            if (slow==fast)
            {
                return true;
            }

            slow = slow->next;
            fast = fast->next->next;
            
        }
        return false;
    }
};

题目来源

• 141. Linked List Cycle