[leetcode] Implement Queue using Stacks

from : https://leetcode.com/problems/implement-queue-using-stacks/
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).

class MyQueue {
Stack<Integer> psh = new Stack<Integer>();
Stack<Integer> pap = new Stack<Integer>();

// Push element x to the back of queue.
public void push(int x) {
psh.push(new Integer(x));
}

// Removes the element from in front of queue.
public void pop() {
if (!pap.isEmpty()) {
pap.pop();
} else {
while (!psh.isEmpty()) {
pap.push(psh.pop());
}
pap.pop();
}
}

// Get the front element.
public int peek() {
if (!pap.isEmpty()) {
return pap.peek();
} else {
while (!psh.isEmpty()) {
pap.push(psh.pop());
}
return pap.peek();
}
}

// Return whether the queue is empty.
public boolean empty() {
return psh.isEmpty() && pap.isEmpty();
}

}