Решение

class Solution {
    fun sumOfLeftLeaves(root: TreeNode?): Int {
        fun dfs(node: TreeNode?, isLeft: Boolean): Int {
            if (node == null) return 0
            val l = node.left
            val r = node.right
            if (l == null && r == null) return if (isLeft) node.`val` else 0
            return dfs(l, true) + dfs(r, false)
        }
        return dfs(root, false)
    }
}