is_balanced.rb

# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val)
#         @val = val
#         @left, @right = nil, nil
#     end
# end

# @param {TreeNode} root
# @return {Boolean}
def is_balanced(root)
    bool, _ = is_balanced?(root)
    bool
end

def is_balanced?(root)
    return [true, 0] if root.nil? 
    return [true, 1] if root.left.nil? && root.right.nil?

    #1) Left subtree of T is balanced
    l, l_depth = is_balanced?(root.left)
    
    #2) Right subtree of T is balanced
    r, r_depth = is_balanced?(root.right)
    
    #3) The difference between heights of left subtree and right subtree is not more than 1.
    # abs prevents errors from left side being smaller
    [r && l && ((l_depth - r_depth).abs <= 1), [l_depth, r_depth].max + 1]
end