Put 4 different New Year's cards into 3 different mailboxes, how many ways are there in which all 3 mailboxes are not empty?
将4张不同的新年贺卡投入3个不同的信箱，则3个信箱都不空的投法有多少种．
<gadget id="calculator">4 * 3 * 2</gadget>
<output>24</output>

<gadget id="calculator">3 * 2 * 1</gadget>
<output>6</output>

<gadget id="calculator">24 / 6</gadget>
<output>4</output>

<gadget id="calculator">4 * 3 * 2 * 1 * 3</gadget>
<output>72</output>

<result>72</result>