232.Implement Queue using Stacks

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Solution 1: accepted

Amortized O(1) operations: pop() and peek(). 

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class MyQueue {
    private Stack<Integer> input;
    private Stack<Integer> output;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        input = new Stack<>();
        output = new Stack<>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        input.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        if (output.isEmpty()) {
            while (!input.isEmpty()) {
                output.push(input.pop());
            }
        }
        return output.isEmpty()? 1 : output.pop();
    }
    
    /** Get the front element. */
    public int peek() { // similar to pop()
        if (output.isEmpty()) {
            while (!input.isEmpty()) {
                output.push(input.pop());
            }
        }
        return output.isEmpty()? 1 : output.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return input.isEmpty() && output.isEmpty();
    }
}