2개 스택으로 큐를 구현해보기

Author: gunkim

algorithm/src/main/java/io/github/gunkim/algorithm/queue/TwoStackQueue.java

@@ -0,0 +1,22 @@
+package io.github.gunkim.algorithm.queue;
+
+import java.util.Stack;
+
+public class TwoStackQueue<T> {
+    private final Stack<T> inStack = new Stack<>();
+    private final Stack<T> outStack = new Stack<>();
+
+    public void add(T value) {
+        inStack.push(value);
+    }
+
+    public T get() {
+        if (outStack.isEmpty()) {
+            while (!inStack.isEmpty()) {
+                T value = inStack.pop();
+                outStack.push(value);
+            }
+        }
+        return outStack.pop();
+    }
+}

algorithm/src/test/java/io/github/gunkim/algorithm/queue/TwoStackQueueTest.java

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+package io.github.gunkim.algorithm.queue;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class TwoStackQueueTest {
+    @Test
+    void 요소가_순서대로_검색되는지_테스트() {
+        TwoStackQueue<Integer> queue = new TwoStackQueue<>();
+        queue.add(1);
+        queue.add(2);
+        queue.add(3);
+
+        assertThat(queue.get()).isEqualTo(1);
+        assertThat(queue.get()).isEqualTo(2);
+        assertThat(queue.get()).isEqualTo(3);
+    }
+
+    @Test
+    void 추가와_조회_작업이_번갈아_수행되는지_테스트() {
+        TwoStackQueue<Integer> queue = new TwoStackQueue<>();
+        queue.add(1);
+        queue.add(2);
+
+        assertThat(queue.get()).isEqualTo(1);
+
+        queue.add(3);
+        assertThat(queue.get()).isEqualTo(2);
+        assertThat(queue.get()).isEqualTo(3);
+    }
+
+    @Test
+    void 빈_큐에서_예외가_발생하는지_테스트() {
+        TwoStackQueue<Integer> queue = new TwoStackQueue<>();
+
+        assertThatThrownBy(queue::get)
+                .isInstanceOf(java.util.EmptyStackException.class);
+    }
+
+    @Test
+    void 큐를_비우고_다시_채운후_조회하는_테스트() {
+        TwoStackQueue<Integer> queue = new TwoStackQueue<>();
+        queue.add(1);
+        queue.add(2);
+        queue.get();
+        queue.get();
+
+        queue.add(3);
+        queue.add(4);
+
+        assertThat(queue.get()).isEqualTo(3);
+        assertThat(queue.get()).isEqualTo(4);
+    }
+}