notes/OJ notes/pages/Leetcode Linked-List-Cycle.md

# Leetcode Linked-List-Cycle

#### 2022-06-14 21:54

---

##### Algorithms:

#algorithm #Floyd_s_cycle_finding_algorithm

##### Data structures:

#DS #linked_list

##### Difficulty:

#leetcode #coding_problem #difficulty_easy

##### Related topics:

##### Links:

- [Link to problem](https://leetcode.com/problems/linked-list-cycle/)

---

### Problem

Given `head`, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the `next` pointer. Internally, `pos` is used to denote the index of the node that tail's `next` pointer is connected to. **Note that `pos` is not passed as a parameter**.

Return `true` _if there is a cycle in the linked list_. Otherwise, return `false`.
**Follow up:** Can you solve it using `O(1)` (i.e. constant) memory?

#### Examples

**Example 1:**

![example1](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png)

```markdown
**Input:** head = [3,2,0,-4], pos = 1
**Output:** true
**Explanation:** There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).
```

**Example 2:**

![example2](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png)

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

**Example 3:**

![example3](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png)

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

#### Constraints

- The number of the nodes in the list is in the range `[0, 104]`.
- `-105 <= Node.val <= 105`
- `pos` is `-1` or a **valid index** in the linked-list.

### Thoughts

> [!summary] > **Algorithm:**
> This is a #Floyd_s_cycle_finding_algorithm.
> This is pretty straightforward, visit [[Floyd's Cycle Finding Algorithm]] for more info

### Solution

O(n)

```cpp
/**
 * 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) {
    ListNode *slow = head;
    ListNode *fast = head;

    while (fast != NULL && fast->next != NULL) {
      slow = slow->next;
      fast = fast->next->next;

      if (slow == fast) {
        return true;
      }
    }
    return false;
  }
};

```