P14 - Duplicate the elements of a list.

> duplicate("abccd".toList())
[a, a, b, b, c, c, c, c, d, d]

kotlin

package org.kotlin99.lists

import com.natpryce.hamkrest.assertion.assertThat
import com.natpryce.hamkrest.equalTo
import org.junit.Test

fun <T> duplicate(list: List<T>): List<T> = list.flatMap { listOf(it, it) }

class P14Test {
    @Test fun `duplicate the elements of a list`() {
        assertThat(duplicate("abccd".toList()), equalTo("aabbccccdd".toList()))
    }
}