141. Linked List Cycle

1.問題

• 判斷linked list是否有cycle

2.想法

• 提問

• function header, parameter

• test input

• 觀察

• 說明想法

  • 快慢指針: 若是有cycle, 則快慢指針一定會colid

• 測試計算複雜度

3.程式碼

/**
 * 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* slow = head;
        ListNode* fast = head->next;
        while (slow && slow->next && 
               fast && fast->next && fast->next->next) {
            if (slow == fast) {
                return true;
            }
            slow = slow->next;
            fast = fast->next->next;
        }
        
        return false;
    }
};