update

leetcode/problem/add-digits.html

@@ -0,0 +1,30 @@
+<p>Given an integer <code>num</code>, repeatedly add all its digits until the result has only one digit, and return it.</p>
+
+<p>&nbsp;</p>
+<p><strong>Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> num = 38
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> The process is
+38 --&gt; 3 + 8 --&gt; 11
+11 --&gt; 1 + 1 --&gt; 2 
+Since 2 has only one digit, return it.
+</pre>
+
+<p><strong>Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> num = 0
+<strong>Output:</strong> 0
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>0 &lt;= num &lt;= 2<sup>31</sup> - 1</code></li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Follow up:</strong> Could you do it without any loop/recursion in <code>O(1)</code> runtime?</p>