题目描述:

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

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

经典题, 使用快慢指针.

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if(!head) return false;
        ListNode *p1 = head, *p2 = head->next;
        while(p1 && p2){
            if(p1 == p2) return true;
            p1 = p1->next;
            if(!p2->next){
                return false;
            }
            p2 = p2->next->next;
        }
        return false;
    }
};