Subsets II
<p>Given an integer array <code>nums</code> that may contain duplicates, return <em>all possible</em> <span data-keyword="subset"><em>subsets</em></span><em> (the power set)</em>.</p>

<p>The solution set <strong>must not</strong> contain duplicate subsets. Return the solution in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> nums = [1,2,2]
<strong>Output:</strong> [[],[1],[1,2],[1,2,2],[2],[2,2]]
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> nums = [0]
<strong>Output:</strong> [[],[0]]
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10</code></li>
	<li><code>-10 &lt;= nums[i] &lt;= 10</code></li>
</ul>
Bit Manipulation; Array; Backtracking
public class Solution {
    private IList<IList<int>> ans = new List<IList<int>>();
    private IList<int> t = new List<int>();
    private int[] nums;

    public IList<IList<int>> SubsetsWithDup(int[] nums) {
        Array.Sort(nums);
        this.nums = nums;
        Dfs(0);
        return ans;
    }

    private void Dfs(int i) {
        if (i >= nums.Length) {
            ans.Add(new List<int>(t));
            return;
        }
        t.Add(nums[i]);
        Dfs(i + 1);
        t.RemoveAt(t.Count - 1);
        while (i + 1 < nums.Length && nums[i + 1] == nums[i]) {
            ++i;
        }
        Dfs(i + 1);
    }
}
