source: https://leetcode.com/problems/linked-list-cycle/
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 = [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.
Solution
I can think of 2 different solutions.
- 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.
- 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.
- Both pointers start at
Solution 1:
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/**
* 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:
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/**
* 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;
}
};