添加 0015.三数之和 C#版本

problems/0015.三数之和.md

@@ -509,5 +509,64 @@ int** threeSum(int* nums, int numsSize, int* returnSize, int** returnColumnSizes
 }
 ```
 
+C#:
+```csharp
+public class Solution
+{
+    public IList<IList<int>> ThreeSum(int[] nums)
+    {
+        var result = new List<IList<int>>();
+
+        Array.Sort(nums);
+
+        for (int i = 0; i < nums.Length - 2; i++)
+        {
+            int n1 = nums[i];
+
+            if (n1 > 0)
+                break;
+
+            if (i > 0 && n1 == nums[i - 1])
+                continue;
+
+            int left = i + 1;
+            int right = nums.Length - 1;
+
+            while (left < right)
+            {
+                int n2 = nums[left];
+                int n3 = nums[right];
+                int sum = n1 + n2 + n3;
+
+                if (sum > 0)
+                {
+                    right--;
+                }
+                else if (sum < 0)
+                {
+                    left++;
+                }
+                else
+                {
+                    result.Add(new List<int> { n1, n2, n3 });
+
+                    while (left < right && nums[left] == n2)
+                    {
+                        left++;
+                    }
+
+                    while (left < right && nums[right] == n3)
+                    {
+                        right--;
+                    }
+                }
+            }
+        }
+
+        return result;
+    }
+}
+```
+
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