LeetCode/kt/first-unique-character-in-a-string.kt

class Solution {
    private data class Accumulator(val firstIndex: Int, var count: Int = 0) {
        fun add() {
            count++
        }
    }

    fun firstUniqChar(s: String): Int =
        s
            .foldIndexed(mutableMapOf<Char, Accumulator>()) { i, acc, c ->
                acc.getOrPut(c) { Accumulator(i) }.add()
                acc
            }
            .filterValues { it.count == 1 }
            .minByOrNull { (_, acc) -> acc.firstIndex }
            ?.value
            ?.firstIndex
            ?: -1
}

fun main() {
    val s = Solution()

    check(s.firstUniqChar("leetcode") == 0)
    check(s.firstUniqChar("loveleetcode") == 2)
    check(s.firstUniqChar("aabb") == -1)
}