141. Linked List Cycle 注意邊界條件

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

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

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這裏要注意 邊界條件由快那個指針確定  不用考慮慢那個  若只考慮快那個一次走兩步的情況 那麼 一次走兩步若next爲空 next->next則無法訪問 

/**
 * 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) {
        //設置兩個指針，一個走兩步一個走一步  有環必然會存在相等的時候
        if(head == NULL){
            return false;
        }
        if(head->next == head){
            return true;
        }
       ListNode*t1 = head;
        ListNode*t2 = head;
        //這裏要注意 邊界條件由快那個指針確定  不用考慮慢那個  若只考慮快那個一次走兩步的情況 那麼 一次走兩步若next爲空 next->next則無法訪問
        while(t2->next!=NULL&&t2->next->next!=NULL){
            t1 = t1->next;
            t2 = t2->next->next;
            if(t1 == t2){
                return true;
            }
        }
        return false;
    }
};