add peek to queue interface

- update travis file
- fix typo

.travis.yml

@@ -1,8 +1,18 @@
 language: java
+
 jdk:
   - oraclejdk8
+  - oraclejdk9
+  - openjdk10
+
+cache:
+  directories:
+    - ~/.gradle/caches/
+    - ~/.gradle/wrapper/
+
 before_install:
   - chmod +x gradlew
   - chmod +x gradle/wrapper/gradle-wrapper.jar
+
 script:
   - ./gradlew test build

src/main/kotlin/it/norangeb/algorithms/datastructures/queue/Queue.kt

@@ -30,6 +30,7 @@ import arrow.core.Option
 interface Queue<T> {
     fun enqueue(elem: T)
     fun dequeue(): Option<T>
+    fun peek(): Option<T>
     fun isEmpty(): Boolean
     fun size(): Int
     fun clean()

src/main/kotlin/it/norangeb/algorithms/datastructures/queue/ResizingArrayQueue.kt

@@ -51,10 +51,11 @@ class ResizingArrayQueue<T>(capacity: Int = DEFAULT_CAPACITY) : Queue<T> {
     }
 
     override fun dequeue(): Option<T> {
-        if (isEmpty())
-            return None
+        val elem = peek()
+
+        if (elem is None)
+            return elem
 
-        val elem = queue[head]
         queue[head] = null
         head = (head + 1) % queue.size
         size--
@@ -62,7 +63,14 @@ class ResizingArrayQueue<T>(capacity: Int = DEFAULT_CAPACITY) : Queue<T> {
         if (isOneQuarterFull())
             resizeArray(queue.size / RESIZE_FACTOR)
 
-        return elem.toOption()
+        return elem
+    }
+
+    override fun peek(): Option<T> {
+        if (isEmpty())
+            return None
+
+        return queue[head].toOption()
     }
 
     private fun resizeArray(capacity: Int) {

src/test/kotlin/it/norangeb/algorithms/datastructures/queue/ResizingArrayQueueTest.kt

@@ -26,6 +26,7 @@
 package it.norangeb.algorithms.datastructures.queue
 
 import arrow.core.None
+import arrow.core.Some
 import arrow.core.getOrElse
 import org.amshove.kluent.`should be equal to`
 import org.junit.jupiter.api.Test
@@ -62,6 +63,16 @@ class ResizingArrayQueueTest {
         queue.size() `should be equal to` 0
     }
 
+    @Test
+    fun testPeek() {
+        val queue = ResizingArrayQueue<Int>()
+
+        (queue.peek() is None) `should be equal to` true
+        queue.enqueue(1)
+        (queue.peek() is Some) `should be equal to` true
+        queue.size() `should be equal to` 1
+    }
+
     @Test
     fun testIncreaseSize() {
         val queue = ResizingArrayQueue<Int>()

src/test/kotlin/it/norangeb/algorithms/datastructures/stack/ResizingArrayStackTest.kt

@@ -71,7 +71,7 @@ class ResizingArrayStackTest {
     }
 
     @Test
-    fun testIncrease() {
+    fun testIncreaseSize() {
         val stack = ResizingArrayStack<Int>()
 
         stack.push(1)
@@ -82,7 +82,7 @@ class ResizingArrayStackTest {
     }
 
     @Test
-    fun testDescrese() {
+    fun testDecreaseSize() {
         val stack = ResizingArrayStack<Int>()
 
         stack.push(1)