# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val = 0, left = nil, right = nil)
#         @val = val
#         @left = left
#         @right = right
#     end
# end
# @param {TreeNode} root
# @return {Integer[]}
def inorder_traversal(root)
    result = []
    return result if root.nil?

    result << inorder_traversal(root.left)
    result << root.val
    result << inorder_traversal(root.right)
    
    result.flatten
end