kt: add «2523. Closest Prime Numbers in Range»

URL:	https://leetcode.com/problems/closest-prime-numbers-in-range/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/closest-prime-numbers-in-range.kt

@@ -0,0 +1,44 @@
+class Solution {
+    private fun isPrime(n: Int): Boolean =
+        when {
+            n < 2 -> false
+            n % 2 == 0 -> n == 2
+            else ->
+                (3..n).step(2)
+                    .asSequence()
+                    .takeWhile { it * it <= n }
+                    .all { div -> n % div != 0 }
+        }
+
+    private data class Acc(val lastPrime: Int, val p: Int, val q: Int, val diff: Int) {
+        constructor() : this(-1, -1, -1, 1_000_000) {}
+
+        val result: IntArray =
+            when {
+                p == -1 -> intArrayOf(-1, -1)
+                else -> intArrayOf(p, q)
+            }
+
+        private fun check(r: Int): Triple<Int, Int, Int>? =
+            when {
+                lastPrime == -1 -> null
+                r - lastPrime >= diff -> null
+                else -> Triple(lastPrime, r, r - lastPrime)
+            }
+
+        fun update(r: Int): Acc =
+            (check(r) ?: Triple(p, q, diff)).let { (p, q, diff) ->
+                copy(lastPrime = r, p = p, q = q, diff = diff)
+            }
+    }
+
+    fun closestPrimes(
+        left: Int,
+        right: Int,
+    ): IntArray =
+        (left..right)
+            .asSequence()
+            .filter { p -> isPrime(p) }
+            .fold(Acc()) { acc, p -> acc.update(p) }
+            .result
+}