feat : stack, queue 구현

```
Stack.java
Queue 두개를 이용해 Stack 구현
Queue.java
Stack 두개를 이용해 Queue 구현

```

Author: cyeji

example/data_structure/Queue.java

@@ -0,0 +1,34 @@
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.util.Arrays;
+import java.util.Stack;
+
+class Queue<T>{
+    Stack<T> inStack;
+    Stack<T> outStack;
+
+    public Queue () {
+        this.inStack = new Stack<>();
+        this.outStack = new Stack<>();
+    }
+
+    public void offer(T obj){
+        this.outStack.push( obj );
+    }
+
+    public T poll(){
+        while(!outStack.isEmpty()) {
+            inStack.push(outStack.pop());
+        }
+        T number = inStack.pop();
+        while(!inStack.isEmpty()) {
+            outStack.push( inStack.pop() );
+        }
+        return number;
+    }
+
+    public boolean isEmpty(){
+        return outStack.isEmpty();
+    }
+}

example/data_structure/Stack.java

@@ -0,0 +1,37 @@
+import java.util.LinkedList;
+import java.util.Queue;
+
+public class Stack<T> {
+
+    Queue<T> inQueue;
+    Queue<T> outQueue;
+
+    public Stack(){
+        this.inQueue = new LinkedList<>();
+        this.outQueue = new LinkedList<>();
+    }
+
+    public void push(T obj){
+        this.outQueue.offer( obj );
+    }
+
+    public T pop(){
+        while ( outQueue.size() != 1){
+            inQueue.offer( outQueue.poll() );
+        }
+        T poll = outQueue.poll();
+        while ( !inQueue.isEmpty() ){
+            outQueue.offer( inQueue.poll() );
+        }
+        return poll;
+    }
+
+    public boolean isEmpty(){
+        return outQueue.isEmpty();
+    }
+
+    public int size(){
+        return outQueue.size();
+    }
+
+}