Update0090.子集2，添加C#

problems/0090.子集II.md

@@ -640,6 +640,31 @@ object Solution {
   }
 }
 ```
+### C#
+```c#
+public class Solution
+{
+    public IList<IList<int>> res = new List<IList<int>>();
+    public IList<int> path = new List<int>();
+    public IList<IList<int>> SubsetsWithDup(int[] nums)
+    {
+        Array.Sort(nums);
+        BackTracking(nums, 0);
+        return res;
+    }
+    public void BackTracking(int[] nums, int start)
+    {
+        res.Add(new List<int>(path));
+        for (int i = start; i < nums.Length; i++)
+        {
+            if (i > start && nums[i] == nums[i - 1]) continue;
+            path.Add(nums[i]);
+            BackTracking(nums, i + 1);
+            path.RemoveAt(path.Count - 1);
+        }
+    }
+}
+```
 
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