[leetcode]: 141. Linked List Cycle

1.題目

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

Follow up:

Can you solve it without using extra space?

給一個連結清單，判斷是否有環

2.分析

關于是否成環的判斷在Happy Number這題就已經用到過floyd cycle detection algorithm

兩個指針slow每次走1步,fast每次走2(or more)步。如果存在cycle，那麼slow和fast必定在某點相遇。

3.代碼

class Solution {
public:
   bool hasCycle(ListNode *head) {
        if (head == NULL || head->next == NULL)
            return false;
        ListNode* slow = head;
        ListNode* fast = head;
        while(slow&&fast&&fast->next)
        {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast)
                return true;
        } 
        return false;
    }
};