/**
 * Example:
 * var ti = TreeNode(5)
 * var v = ti.`val`
 * Definition for a binary tree node.
 * class TreeNode(var `val`: Int) {
 *     var left: TreeNode? = null
 *     var right: TreeNode? = null
 * }
 */
class Solution {
    fun inorderTraversal(root: TreeNode?, values: MutableList<Int>): List<Int> {
        if (root == null) {
            return values
        }

        inorderTraversal(root.left, values)
        values.add(root.`val`!!)
        inorderTraversal(root.right, values)

        return values
    }

    fun inorderTraversal(root: TreeNode?): List<Int>
        = inorderTraversal(root, mutableListOf<Int>())
}