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'snext
pointer is connected to. Note thatpos
is not passed as a parameter.Return
true
if there is a cycle in the linked list. Otherwise, returnfalse
.
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.
Pseudocode
traversing a cylical graph with a single pointer results in an infinite loop as there is not terminating null nodes.
if two pointers are used, with one of the pointer travelling twice as fast as the other, at some point these two pointers will meet.
Solution
var hasCycle = function (head) {
let fast = head;
let slow = head;
while (fast !== null && fast.next !== null) {
fast = fast.next.next;
slow = slow.next;
if (fast === slow) {
return true;
}
}
return false;
};
Time and Space Complexity
Time
A while loop that terminates when there's a null node, or when fast = slow. Complete traversal of the linked list O(N)
Total - O(N)
Space
Memory requirements are constant as pointer assignments are constant time O(1)
Total - O(1)
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