141 Linked List Cycle

Problem Statement

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.

Input: head = [3,2,0,-4], pos = 1
Output: true

Approach

There is two possible approaches:

• Using two pointers and a Set. BigO(N).
• Slow and Fast Pointers. BigO(1)

Solution

// First solution
function hasCycle(head) {
  let set = new Set()
  let current = head
  while (current) {
    if (set.has(current.val)) {
      return true
    }

    set.add(current.val)
    current = current.next
  }

  return false
}

// Second Solution
var hasCycle = function(head) {
	let slow = head
	let fast = head

	while (fast && fast.next) {
		fast = fast.next.next
		slow = slow.next
		if (fast === slow) {
			return true
		}
	}

	return false
};

Questions

References

LC

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