problems: add „13. Roman to Integer“

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

problems/roman-to-integer.kt

@@ -0,0 +1,62 @@
+class Solution {
+    /**
+     * Holds fixed mapping of roman numerals to their respective numeric value.
+     */
+    private val values = mapOf(
+        'I' to 1, 'V' to 5, 'X' to 10, 'L' to 50, 'C' to 100, 'D' to 500, 'M' to 1000
+    )
+
+    /**
+     * Represents an accumulator for right fold when turning roman numeral into an integer value.
+     * @property sum Holds the current sum of the accumulator.
+     * @property next Holds the value of the previous roman numeral digit.
+     */
+    private data class Accumulator(val sum: Int = 0, val next: Int = 0) {
+        /**
+         * @param it Current roman numeral digit.
+         * @return Sign that is to be used when modifying sum of the accumulator.
+         */
+        private fun sign(it: Int): Int = when (next > it) {
+            true -> -1
+            false -> 1
+        }
+
+        /**
+         * @param it Currently processed roman numeral digit.
+         * @return Correctly updated accumulator.
+         */
+        fun update(it: Int): Accumulator = copy(sum = sum + sign(it) * it, next = it)
+    }
+
+    fun romanToInt(s: String): Int = s
+        .map { values[it]!! }
+        .foldRight(Accumulator()) { it, acc ->
+            acc.update(it)
+        }
+        .sum
+}
+
+fun main() {
+    val s = Solution()
+
+    val tests = mapOf(
+        "I" to 1,
+        "II" to 2,
+        "III" to 3,
+        "IV" to 4,
+        "V" to 5,
+        "VI" to 6,
+        "VII" to 7,
+        "VIII" to 8,
+        "IX" to 9,
+        "X" to 10,
+        "LVIII" to 58,
+        "MCMXCIV" to 1994
+    )
+
+    tests.forEach { (num, i) ->
+        check(s.romanToInt(num) == i) {
+            "$num: ${s.romanToInt(num)} == $i"
+        }
+    }
+}