leetcode-problemset/leetcode-cn/problem (English)/三步问题(English) [three-steps-problem-lcci].html

<p>A child is running up a staircase with n steps and can hop either 1 step, 2 steps, or 3 steps at a time. Implement a method to count how many possible ways the child can run up the stairs.&nbsp;The result may be large, so return it modulo 1000000007.</p>

<p><strong>Example1:</strong></p>

<pre>
<strong> Input</strong>: n = 3 
<strong> Output</strong>: 4
</pre>

<p><strong>Example2:</strong></p>

<pre>
<strong> Input</strong>: n = 5
<strong> Output</strong>: 13
</pre>

<p><strong>Note:</strong></p>

<ol>
	<li><code>1 &lt;= n &lt;= 1000000</code></li>
</ol>