Решение

class Solution {
    fun isBalanced(root: TreeNode?): Boolean = height(root) != -1
    private fun height(node: TreeNode?): Int {
        if (node == null) return 0
        val l = height(node.left); if (l == -1) return -1
        val r = height(node.right); if (r == -1) return -1
        if (kotlin.math.abs(l - r) > 1) return -1
        return if (l > r) l + 1 else r + 1
    }
}