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142. Linked List Cycle II
Medium
Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.
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 (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter.
Do not modify the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: no cycle
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 <= 105posis-1or a valid index in the linked-list.
Follow up: Can you solve it using O(1) (i.e. constant) memory?
Solution
# Definition for singly-linked list.
# class ListNode
# attr_accessor :val, :next
# def initialize(val)
# @val = val
# @next = nil
# end
# end
# @param {ListNode} head
# @return {ListNode}
def detectCycle(head)
return nil if head.nil? || head.next.nil?
slow = head
fast = head
while fast && fast.next
fast = fast.next.next
slow = slow.next
# Intersected inside the loop.
break if slow == fast
end
return nil if fast.nil? || fast.next.nil?
slow = head
while slow != fast
slow = slow.next
fast = fast.next
end
slow
end