leetcode: Implement Queue using Stacks

用两个栈来模拟队列，一个负责记录入栈顺序，另一个记录这个的反顺序，即队列的正常顺序...push插入input栈，O(1); pop和peek看output栈，当output为空的时候就将input内的所有数据导入过来即可....

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

    // Removes the element from in front of queue.
    public void pop() {
        if( output.size()==0 )
        {
            while( input.size()!=0 )
            {
                output.push( input.pop() );
            }
        }
        output.pop();
    }

    // Get the front element.
    public int peek() {
        if( output.size()==0 )
        {
            while( input.size()!=0 )
            {
                output.push( input.pop() );
            }
        }
        return output.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return output.empty()&&input.empty();
        
    }
}