[Algorithm] 11. Linked List Cycle
Description
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
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.
Challenge
Follow up:
Can you solve it without using extra space? (O(1) (i.e. constant) memory)?
Solution
/**
* 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==NULL) return false; ListNode *fast = head;
ListNode *slow = head; while(true){
// If a node is NULL, there is no cycle.
if(fast->next == NULL || fast->next->next == NULL) return false; slow = slow->next;
fast = fast->next->next; // When the fast and the slow run into the same node, there's a cycle.
if ( slow->val == fast->val )
return true;
}
}
};
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