232. Implement Queue using Stacks

, 
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class MyQueue {

    private final Stack<Integer> inStack;
    private final Stack<Integer> outStack;

    public MyQueue() {
        // queue: 1 2 3

        //    3       1
        //    2       2
        //    1       3
        // inStack outStack

        this.inStack = new Stack<>();
        this.outStack = new Stack<>();
    }

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

    public int pop() {
        migrateElements();
        return outStack.pop();
    }

    private void migrateElements() {
        if (outStack.isEmpty()) {
            while (!inStack.empty()) {
                outStack.push(inStack.pop());
            }
        }
    }

    public int peek() {
        migrateElements();
        return outStack.peek();
    }

    public boolean empty() {
        return inStack.isEmpty() && outStack.isEmpty();
    }

}

References

232. Implement Queue using Stacks
剑指 Offer 09. 用两个栈实现队列