feat: add cs solution to lc problem: No.1814 (#1997)

Author: dev-mauli

solution/1800-1899/1814.Count Nice Pairs in an Array/README.md

@@ -322,6 +322,35 @@ function countNicePairs(nums: number[]): number {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int CountNicePairs(int[] nums) {
+        Dictionary<int, int> cnt = new Dictionary<int, int>();
+        foreach (int x in nums) {
+            int y = x - Rev(x);
+            cnt[y] = cnt.GetValueOrDefault(y, 0) + 1;
+        }
+        int mod = (int)1e9 + 7;
+        long ans = 0;
+        foreach (int v in cnt.Values) {
+            ans = (ans + (long)v * (v - 1) / 2) % mod;
+        }
+        return (int)ans;
+    }
+
+    private int Rev(int x) {
+        int y = 0;
+        while (x > 0) {
+            y = y * 10 + x % 10;
+            x /= 10;
+        }
+        return y;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1800-1899/1814.Count Nice Pairs in an Array/README_EN.md

@@ -314,6 +314,35 @@ function countNicePairs(nums: number[]): number {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int CountNicePairs(int[] nums) {
+        Dictionary<int, int> cnt = new Dictionary<int, int>();
+        foreach (int x in nums) {
+            int y = x - Rev(x);
+            cnt[y] = cnt.GetValueOrDefault(y, 0) + 1;
+        }
+        int mod = (int)1e9 + 7;
+        long ans = 0;
+        foreach (int v in cnt.Values) {
+            ans = (ans + (long)v * (v - 1) / 2) % mod;
+        }
+        return (int)ans;
+    }
+
+    private int Rev(int x) {
+        int y = 0;
+        while (x > 0) {
+            y = y * 10 + x % 10;
+            x /= 10;
+        }
+        return y;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1800-1899/1814.Count Nice Pairs in an Array/Solution.cs

@@ -0,0 +1,24 @@
+public class Solution {
+    public int CountNicePairs(int[] nums) {
+        Dictionary<int, int> cnt = new Dictionary<int, int>();
+        foreach (int x in nums) {
+            int y = x - Rev(x);
+            cnt[y] = cnt.GetValueOrDefault(y, 0) + 1;
+        }
+        int mod = (int)1e9 + 7;
+        long ans = 0;
+        foreach (int v in cnt.Values) {
+            ans = (ans + (long)v * (v - 1) / 2) % mod;
+        }
+        return (int)ans;
+    }
+
+    private int Rev(int x) {
+        int y = 0;
+        while (x > 0) {
+            y = y * 10 + x % 10;
+            x /= 10;
+        }
+        return y;
+    }
+}