feat: add cs solution to lc problem: No.1887 (#1984)

Author: dev-mauli

solution/1800-1899/1887.Reduction Operations to Make the Array Elements Equal/README.md

@@ -201,6 +201,24 @@ func reductionOperations(nums []int) int {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int ReductionOperations(int[] nums) {
+        Array.Sort(nums);
+        int ans = 0, up = 0;
+        for (int i = 1; i < nums.Length; i++) {
+            if (nums[i] != nums[i - 1]) {
+                up++;
+            }
+            ans += up;
+        }
+        return ans;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1800-1899/1887.Reduction Operations to Make the Array Elements Equal/README_EN.md

@@ -183,6 +183,24 @@ func reductionOperations(nums []int) int {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int ReductionOperations(int[] nums) {
+        Array.Sort(nums);
+        int ans = 0, up = 0;
+        for (int i = 1; i < nums.Length; i++) {
+            if (nums[i] != nums[i - 1]) {
+                up++;
+            }
+            ans += up;
+        }
+        return ans;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1800-1899/1887.Reduction Operations to Make the Array Elements Equal/Solution.cs

@@ -0,0 +1,13 @@
+public class Solution {
+    public int ReductionOperations(int[] nums) {
+        Array.Sort(nums);
+        int ans = 0, up = 0;
+        for (int i = 1; i < nums.Length; i++) {
+            if (nums[i] != nums[i - 1]) {
+                up++;
+            }
+            ans += up;
+        }
+        return ans;
+    }
+}