add leetcode problem-cn part2

算法题（国内版）/problem (English)/幂集(English) [power-set-lcci].html

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+<p>Write a method to return all subsets of a set. The elements in a set are&nbsp;pairwise distinct.</p>
+
+<p>Note: The result set should not contain duplicated subsets.</p>
+
+<p><strong>Example:</strong></p>
+
+<pre>
+<strong> Input</strong>:  nums = [1,2,3]
+<strong> Output</strong>: 
+[
+  [3],
+&nbsp; [1],
+&nbsp; [2],
+&nbsp; [1,2,3],
+&nbsp; [1,3],
+&nbsp; [2,3],
+&nbsp; [1,2],
+&nbsp; []
+]
+</pre>