[LeetCode] Problem 232 - Implement Queue using Stacks

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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.

Example

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MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).

Code

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private Stack<Integer> stack1;
private Stack<Integer> stack2;
private int top;

public MyQueue() {
    stack1 = new Stack<>();
    stack2 = new Stack<>();
}

public void push(int x) {
    if (stack1.isEmpty())
        top = x;

    stack1.push(x);
}

public int pop() {
    if (stack2.isEmpty()) {
        while (!stack1.isEmpty())
            stack2.push(stack1.pop());
    }

    return stack2.pop();
}

public int peek() {
    if (stack2.isEmpty())
        return top;

    return stack2.peek();
}

public boolean empty() {
    return stack1.isEmpty() && stack2.isEmpty();
}