<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. 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 <= n <= 1000000</code></li> </ol>