Add solution for Day 3

src/Day03.kt

@@ -0,0 +1,53 @@
+fun main() {
+    fun part1(input: List<String>): Int {
+        var priority = 0
+        input.forEach { ruckSack ->
+            val compartment1 = ruckSack.slice(0 until ruckSack.length / 2).toSet()
+            val compartment2 = ruckSack.slice(ruckSack.length / 2 until ruckSack.length).toSet()
+            var duplicate: Char? = null
+            for (item in compartment1) {
+                if (compartment2.contains(item)) {
+                    duplicate = item
+                    break
+                }
+            }
+            duplicate?.let {
+                priority += it.priority()
+            }?: throw IllegalStateException("No duplicates found in rucksacks")
+        }
+        return priority
+    }
+
+    fun part2(input: List<String>): Int {
+        var priority = 0
+        val group = mutableListOf<Set<Char>>()
+        input.forEach { ruckSack ->
+            group.add(ruckSack.toSet())
+            if (group.size != 3) {
+                return@forEach
+            }
+            for (item in group.first()) {
+                if (group[1].contains(item) && group[2].contains(item)) {
+                    priority += item.priority()
+                    break
+                }
+            }
+            group.clear()
+        }
+        return priority
+    }
+
+    val testInput = readInput("Day03_test")
+    check(part1(testInput) == 157)
+    check(part2(testInput) == 70)
+
+    val input = readInput("Day03")
+    println(part1(input))
+    println(part2(input))
+}
+
+fun Char.priority(): Int = when(code) {
+    in 65..90 -> code - 38
+    in 97..122 -> code - 96
+    else -> throw IllegalArgumentException("Invalid item")
+}