feat: add cs solution to lc problem: No.1877 (doocs#1978)

Author: dev-mauli

solution/1800-1899/1877.Minimize Maximum Pair Sum in Array/README.md

@@ -136,6 +136,21 @@ function minPairSum(nums: number[]): number {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int MinPairSum(int[] nums) {
+        Array.Sort(nums);
+        int ans = 0, n = nums.Length;
+        for (int i = 0; i < n >> 1; ++i) {
+            ans = Math.Max(ans, nums[i] + nums[n - i - 1]);
+        }
+        return ans;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1800-1899/1877.Minimize Maximum Pair Sum in Array/README_EN.md

@@ -122,6 +122,21 @@ function minPairSum(nums: number[]): number {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int MinPairSum(int[] nums) {
+        Array.Sort(nums);
+        int ans = 0, n = nums.Length;
+        for (int i = 0; i < n >> 1; ++i) {
+            ans = Math.Max(ans, nums[i] + nums[n - i - 1]);
+        }
+        return ans;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1800-1899/1877.Minimize Maximum Pair Sum in Array/Solution.cs

@@ -0,0 +1,10 @@
+public class Solution {
+    public int MinPairSum(int[] nums) {
+        Array.Sort(nums);
+        int ans = 0, n = nums.Length;
+        for (int i = 0; i < n >> 1; ++i) {
+            ans = Math.Max(ans, nums[i] + nums[n - i - 1]);
+        }
+        return ans;
+    }
+}