# 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 swap_pairs(head)

if head.nil? or head.next.nil?

return head

end

second_node = head

head = head.next

third_node = head.next

head.next = second_node

second_node.next = (third_node.nil? or third_node.next.nil?) ? third_node : third_node.next

until third_node.nil? do

ptr = third_node

second_node = ptr.next

if second_node.nil?

return head

end

third_node = second_node.next

second_node.next = ptr

ptr.next = (third_node.nil? or third_node.next.nil?) ? third_node : third_node.next

end

head

end

head = ListNode.new 1

head.next = ListNode.new 2

head.next.next = ListNode.new 3

head.next.next.next = ListNode.new 4

head = swap_pairs head

ptr = head

until ptr.nil?

print "#{ptr.val}->"

ptr = ptr.next

end