span> <span class="n">tid</span><span class="p">:</span>
                <span class="n">last</span> <span class="o">=</span> <span class="n">mid</span> <span class="o">-</span> <span class="mi">1</span>
            <span class="k">else</span><span class="p">:</span>
                <span class="k">return</span> <span class="n">List</span><span class="p">[</span><span class="n">mid</span><span class="p">]</span>
        <span class="k">return</span> <span class="kc">None</span>

    <span class="k">def</spa