5. Очередь на двух стеках [#160 #182061]

src/main/java/ru/j4j/collection/SimpleQueue.java

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+package ru.j4j.collection;
+
+import java.util.NoSuchElementException;
+
+public class SimpleQueue<T> {
+    private final SimpleStack<T> input = new SimpleStack<>();
+    private final SimpleStack<T> output = new SimpleStack<>();
+    int size;
+
+    public T poll() {
+        if (size == 0) {
+            throw new NoSuchElementException("Queue is empty");
+        }
+        for (int i = 0; i < size; i++) {
+            output.push(input.pop());
+        }
+        T result = output.pop();
+        size--;
+        for (int i = 0; i < size; i++) {
+            input.push(output.pop());
+        }
+        return result;
+    }
+
+    public void push(T value) {
+        input.push(value);
+        size++;
+    }
+}

src/main/java/ru/j4j/collection/SimpleStack.java

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+package ru.j4j.collection;
+
+public class SimpleStack<T> {
+
+    private final ForwardLinked<T> linked = new ForwardLinked<>();
+
+    public T pop() {
+        return linked.deleteFirst();
+    }
+
+    public void push(T value) {
+        linked.addFirst(value);
+    }
+}

src/test/java/ru/j4j/collection/SimpleQueueTest.java

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+package ru.j4j.collection;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.NoSuchElementException;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class SimpleQueueTest {
+    private SimpleQueue<Integer> queue;
+
+    @BeforeEach
+    public void setUp() {
+        queue = new SimpleQueue<>();
+        queue.push(1);
+    }
+
+    @Test
+    void whenPushPoll() {
+        assertThat(queue.poll()).isEqualTo(1);
+    }
+
+    @Test
+    void when2PushPoll() {
+        queue.push(2);
+        assertThat(queue.poll()).isEqualTo(1);
+    }
+
+    @Test
+    void when2PushPollPushPoll() {
+        queue.poll();
+        queue.push(2);
+        assertThat(queue.poll()).isEqualTo(2);
+    }
+
+    @Test
+    void whenEmptyPoll() {
+        SimpleQueue<Integer> queue = new SimpleQueue<>();
+        assertThatThrownBy(queue::poll)
+                .isInstanceOf(NoSuchElementException.class)
+                .hasMessageContaining("Queue is empty");
+    }
+
+    @Test
+    void when2PushPollPushPollEmpty() {
+        queue.poll();
+        queue.push(2);
+        queue.poll();
+        assertThatThrownBy(queue::poll)
+                .isInstanceOf(NoSuchElementException.class)
+                .hasMessageContaining("Queue is empty");
+    }
+
+    @Test
+    void whenPushPushPollAndPush() {
+        queue.push(2);
+        queue.poll();
+        queue.push(3);
+        assertThat(queue.poll()).isEqualTo(2);
+    }
+}

src/test/java/ru/j4j/collection/SimpleStackTest.java

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+package ru.j4j.collection;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.NoSuchElementException;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+class SimpleStackTest {
+    private SimpleStack<Integer> stack;
+
+    @BeforeEach
+    void init() {
+        stack = new SimpleStack<>();
+        stack.push(1);
+        stack.push(2);
+    }
+
+    @Test
+    void whenPushThenPoll() {
+        stack.push(3);
+        stack.push(4);
+        assertThat(stack.pop()).isEqualTo(4);
+    }
+
+    @Test
+    void whenPushPollThenPushPoll() {
+        stack.pop();
+        stack.push(3);
+        assertThat(stack.pop()).isEqualTo(3);
+    }
+
+    @Test
+    void whenPushPushThenPollPoll() {
+        stack.pop();
+        assertThat(stack.pop()).isEqualTo(1);
+        assertThatThrownBy(stack::pop)
+                .isInstanceOf(NoSuchElementException.class);
+    }
+}