Решение

/**
 * 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
 * }
 */
fun findSecondMinimumValue(root: TreeNode?): Int {
    if (root == null) return -1

    var secondMin = Long.MAX_VALUE
    val minVal = root.`val`

    fun dfs(node: TreeNode?) {
        if (node == null) return

        if (node.`val` > minVal && node.`val` < secondMin) {
            secondMin = node.`val`.toLong() // Обновляем второе минимальное значение
        } else if (node.`val` == minVal) {
            dfs(node.left)
            dfs(node.right)
        }
    }

    dfs(root)
    return if (secondMin == Long.MAX_VALUE) -1 else secondMin.toInt()
}