Flattening Nested Lists

Part of the Logic & Flow section of Coddy's Kotlin journey. Lesson 19 of 63.

flatten concatenates the inner lists into one list, in outer-row order and then inner-element order. It removes one level of nesting but does not remove duplicates or sort the values.

Inside main:

val rows = listOf(listOf(3, 1), emptyList(), listOf(1, 2))
println(rows.flatten())

The row order is preserved, including both copies of one.

flatten removes one collection nesting level and preserves element order.

Challenge

Medium

Complete flattenRows with parameters grid: List<List<Int>>. Return all row values as one list in row order.

Unless stricter bounds are stated above, collections contain at most 100 elements at each level, and integer arguments and integer collection values are between -1000 and 1000. Text supplied for parsing can include invalid or out-of-range representations as described.

Return a value of type List<Int>. Keep the supplied input/output code. It reads scalar arguments one per line; a list starts with its count followed by its elements, and a map starts with its entry count followed by each key and value. Nested lists repeat the count-and-elements format for each row. The supplied main prints the return value followed by one newline. Lists use Kotlin's standard bracketed format; print no additional labels.

Try it yourself

fun flattenRows(grid: List<List<Int>>): List<Int> {
    // Write your solution here.
    return emptyList()
}
fun main() {
    val gridCount = readln().toInt()
    val grid = mutableListOf<List<Int>>()
    for (i0 in 0 until gridCount) {
        val gridItemCount = readln().toInt()
        val gridItem = mutableListOf<Int>()
        for (i1 in 0 until gridItemCount) {
            val gridItemItem = readln().toInt()
            gridItem.add(gridItemItem)
        }
        grid.add(gridItem)
    }
    println(flattenRows(grid))
}