[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();}}