Add solution for day 1

src/Day01.kt

@@ -1,17 +1,77 @@
 fun main() {
     fun part1(input: List<String>): Int {
-        return input.size
+        return input.sumOf {
+            var firstDigit: Char? = null
+            var lastDigit: Char? = null
+            it.forEach { c ->
+                if (!c.isDigit()) return@forEach
+                if (firstDigit == null) {
+                    firstDigit = c
+                }
+                lastDigit = c
+            }
+            "$firstDigit$lastDigit".toInt()
+        }
     }
 
+    val numbers = listOf(
+        "one",
+        "two",
+        "three",
+        "four",
+        "five",
+        "six",
+        "seven",
+        "eight",
+        "nine",
+        "1",
+        "2",
+        "3",
+        "4",
+        "5",
+        "6",
+        "7",
+        "8",
+        "9"
+    )
+
     fun part2(input: List<String>): Int {
-        return input.size
+        return input.sumOf { line ->
+            var firstIndex: Int = Integer.MAX_VALUE
+            var firstNumberIndex: Int? = null
+            var lastIndex: Int = -1
+            var lastNumberIndex: Int? = null
+            numbers.forEachIndexed { numberIndex, number ->
+                val firstLineIndex = line.indexOf(number)
+                val lastLineIndex = line.lastIndexOf(number)
+                if (firstLineIndex > -1 && firstIndex > firstLineIndex) {
+                    firstIndex = firstLineIndex
+                    firstNumberIndex = numberIndex
+                }
+                if (lastLineIndex > -1 && lastIndex < lastLineIndex) {
+                    lastIndex = lastLineIndex
+                    lastNumberIndex = numberIndex
+                }
+            }
+            "${firstNumberIndex!!.numberValue}${lastNumberIndex!!.numberValue}".toInt()
+        }
     }
 
     // test if implementation meets criteria from the description, like:
     val testInput = readInput("Day01_test")
-    check(part1(testInput) == 1)
+    check(part2(testInput) == 281)
 
     val input = readInput("Day01")
     part1(input).println()
     part2(input).println()
 }
+
+val Int.numberValue: Int
+    get() {
+        val value = (this + 1) % 9
+        return if (value == 0) {
+            9
+        } else {
+            value
+        }
+    }