Given the root to a binary search tree, find the second largest node in the tree.

daily-coding-problem-36.rb

#!/usr/bin/env ruby 

class Node
    attr_accessor :val, :left, :right
    def initialize(val, left=nil, right=nil)
        @val = val 
        @left = left 
        @right = right
    end
end

def largest(node)
    while node 
        return node.val if not node.right
        node = node.right
    end 
end 

def second(node)
    invalid = node.nil? or (node.left.nil? && node.right.nil?)
    puts "Tree must have at least 2 nodes" if invalid
    while !node.nil? 
        return largest(node.left) if node.left and not node.right 
        return node.val if node.right and not node.right.left and not node.right.right 
        node = node.right
    end 
end 

tree = Node.new('A', Node.new('B'), Node.new('C', left=Node.new('D',Node.new('E'))))
puts second(tree)