232. Implement Queue using Stacks

#### QUESTION:

Implement the following operations of a queue using stacks.

• push(x) – Push element x to the back of queue.
• pop() – Removes the element from in front of queue.
• peek() – Get the front element.
• empty() – Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack – which means only `push to top`, `peek/pop from top`, `size`, and `is empty` operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

#### SOLUTION:

``````
public static class MyQueue {
private Stack<Integer> stack;
/** Initialize your data structure here. */
public MyQueue() {
stack = new Stack();
}

/** Push element x to the back of queue. */
public void push(int x) {
stack.push(x);
}

/** Removes the element from in front of queue and returns that element. */
public int pop() {
Iterator<Integer> iterator = stack.iterator();
int tmp = -1;
if(iterator.hasNext()) {
tmp = iterator.next();
iterator.remove();
}
return tmp;
}

/** Get the front element. */
public int peek() {
Iterator<Integer> iterator = stack.iterator();
int tmp = -1;
if (iterator.hasNext()){
tmp = iterator.next();
}
return tmp;
}

/** Returns whether the queue is empty. */
public boolean empty() {
return stack.isEmpty();
}
}

public class MyQueue {
Stack<Integer> input = new Stack();
Stack<Integer> output = new Stack();

public void push(int x) {
input.push(x);
}

public int pop() {
peek();
return output.pop();
}

public int peek() {
if (output.empty())
while (!input.empty())
output.push(input.pop());
return output.peek();
}

public boolean empty() {
return input.empty() && output.empty();
}

}
``````