problems: add „387. First Unique Character in a String“

Signed-off-by: Matej Focko <mfocko@redhat.com>

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

@@ -0,0 +1,26 @@
+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)
+}