add leetcode problem-cn part2

算法题（国内版）/problem (English)/三步问题(English) [three-steps-problem-lcci].html

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+<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>