LeetCode/kt/iterator-for-combination.kt

class CombinationIterator(val characters: String, val combinationLength: Int) {
    private var indices = (0 until combinationLength).toMutableList()

    private fun nextCombination(index: Int) {
        // bump the current index
        indices[index]++

        // we have gone through the whole string
        while (indices[index] == characters.length && index > 0) {
            // bump the previous index
            nextCombination(index - 1)

            // set current index to previous + 1, since it must be the following one
            indices[index] = indices[index - 1] + 1
        }
    }

    // By default bumping just the last index
    private fun nextCombination() = nextCombination(combinationLength - 1)

    private fun getCombination(): String =
        indices.map { characters[it] }.joinToString(separator="")

    fun next(): String {
        // construct the combination
        val combination = getCombination()

        // bump the combination
        nextCombination()

        return combination
    }

    fun hasNext(): Boolean =
        indices.all {
            it < characters.length
        }

}

/**
 * Your CombinationIterator object will be instantiated and called as such:
 * var obj = CombinationIterator(characters, combinationLength)
 * var param_1 = obj.next()
 * var param_2 = obj.hasNext()
 */