栈和队列：用一个栈实现另一个栈的排序

    public Stack<Integer> sortStackByStack(Stack<Integer> stack)
    {
        Stack<Integer> help = new Stack<Integer>();
        while(!stack.isEmpty())
        {
            int tmp = stack.pop();
            while(!help.isEmpty() && tmp < help.peek())
            {
                stack.push(help.pop());
            }
            help.push(tmp);
        }

        return help;
    }