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# LeetCode #141: Linked List Cycle (C/C++).

easy

C/C++ Solution to LeetCode problem 141. 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.

## Examples

### Example 1:

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:

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:

Input: head = , 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.

## Solution

I can think of 2 different solutions.

1. Using a hashmap
• Add the pointer address of the current node to the hashmap.
• If node->next address is already in the hashmap then there is a cycle.
• Move to the next node and repeat.
2. Using two pointers
• Both pointers start at head.
• One pointer will move one node at the time (i.e. node->next), and the other pointer two nodes (i.e. node->next->next).
• As long as the two pointers are not ptrnull, if there is a cycle, both pointers will be equal at some point.
• If any pointer becomes nullptr then there is NO cycle.

### Solution 1:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 /** * 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 *ptr = head; unordered_map<ListNode*, bool> hash_table; hash_table[head] = 1; while (ptr) { if (hash_table[ptr->next]) return true; hash_table[ptr->next] = true; ptr = ptr->next; } return false; } }; 

### Solution 2:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 /** * 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* f = head; ListNode* s = head; while (f && f->next) { s = s->next; f = f->next->next; if (f == s) return true; } return false; } };