Linked List Cycle

Q:

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

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

A:

First, use Set.

/**

* 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) {

unordered_set<ListNode*> S;

ListNode* runner = head;

while(runner != nullptr){

if(S.contains(runner)){

return true;

}

S.insert(runner);

runner = runner->next;

}

return false;

}

};

Two pointer technique

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

follow up:  without extra space
那就用已给的空间了。 例如用head节点（和head.next） 可是，如何防止被我们变换的node不是重复节点呢？
“you could record the ith(i = 1,2,4,8..........) ListNode* and compare it to your iterating ListNode*"





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