binary_tree_valid.rb

# Check if a binary tree is valid
class Solution
    def initialize()
        @prev = nil
    end
    def is_valid_bst(root)
        return true if root.nil?
        left = is_valid_bst(root.left) 
	    return false if (@prev && (@prev >= root.val ))
	    

        @prev = root.val
        right = is_valid_bst(root.right)
        left && right
    end
end