Leetcode: 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).

Understand the problem:
The problem can be solved by using two stacks, stack1 and stack2.
  -- push(x), push an element into stack1. O(1) time complexity.
  -- pop(), check if the stack 2 is empty. If not, pop from stack2. If yes, pop all elements from stack1 and push into stack2. Then pop from stack2. Amortized time complexity is O(1).
  -- peek(). The same as pop().
  -- isEmpty(). Check if both stack1 and stack2 are empty.

Code (Java):

class MyQueue {
    private Stack<Integer> stack1;
    private Stack<Integer> stack2;
    
    public MyQueue() {
        this.stack1 = new Stack<Integer>();
        this.stack2 = new Stack<Integer>();
    }
    // Push element x to the back of queue.
    public void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        if (!stack2.isEmpty()) {
            stack2.pop();
        } else {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
            stack2.pop();
        }
    }

    // Get the front element.
    public int peek() {
        int ret = 0;
        if (!stack2.isEmpty()) {
            ret = stack2.peek();
        } else {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
            ret = stack2.peek();
        }
        
        return ret;
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}