添加 0018.四数之和 C#版本

problems/0018.四数之和.md

@@ -403,5 +403,67 @@ func fourSum(_ nums: [Int], _ target: Int) -> [[Int]] {
 }
 ```
 
+C#:
+```csharp
+public class Solution
+{
+    public IList<IList<int>> FourSum(int[] nums, int target)
+    {
+        var result = new List<IList<int>>();
+
+        Array.Sort(nums);
+
+        for (int i = 0; i < nums.Length - 3; i++)
+        {
+            int n1 = nums[i];
+            if (i > 0 && n1 == nums[i - 1])
+                continue;
+
+            for (int j = i + 1; j < nums.Length - 2; j++)
+            {
+                int n2 = nums[j];
+                if (j > i + 1 && n2 == nums[j - 1])
+                    continue;
+
+                int left = j + 1;
+                int right = nums.Length - 1;
+
+                while (left < right)
+                {
+                    int n3 = nums[left];
+                    int n4 = nums[right];
+                    int sum = n1 + n2 + n3 + n4;
+
+                    if (sum > target)
+                    {
+                        right--;
+                    }
+                    else if (sum < target)
+                    {
+                        left++;
+                    }
+                    else
+                    {
+                        result.Add(new List<int> { n1, n2, n3, n4 });
+
+                        while (left < right && nums[left] == n3)
+                        {
+                            left++;
+                        }
+
+                        while (left < right && nums[right] == n4)
+                        {
+                            right--;
+                        }
+                    }
+                }
+            }
+        }
+
+        return result;
+    }
+}
+```
+
 -----------------------
 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>