feat: add csharp solution to lc problem: No.0009 (#4020)

Author: djvelimir

solution/0000-0099/0009.Palindrome Number/README.md

@@ -210,6 +210,27 @@ var isPalindrome = function (x) {
 };
 ```
 
+#### C#
+
+```cs
+public class Solution
+{
+    public bool IsPalindrome(int x)
+    {
+        if (x < 0 || (x > 0 && x % 10 == 0))
+        {
+            return false;
+        }
+        int y = 0;
+        for (; y < x; x /= 10)
+        {
+            y = y * 10 + x % 10;
+        }
+        return x == y || x == y / 10;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0009.Palindrome Number/README_EN.md

@@ -202,6 +202,27 @@ var isPalindrome = function (x) {
 };
 ```
 
+#### C#
+
+```cs
+public class Solution
+{
+    public bool IsPalindrome(int x)
+    {
+        if (x < 0 || (x > 0 && x % 10 == 0))
+        {
+            return false;
+        }
+        int y = 0;
+        for (; y < x; x /= 10)
+        {
+            y = y * 10 + x % 10;
+        }
+        return x == y || x == y / 10;
+    }
+}
+```
+
 #### PHP
 
 ```php

solution/0000-0099/0009.Palindrome Number/Solution.cs

@@ -0,0 +1,16 @@
+public class Solution
+{
+    public bool IsPalindrome(int x)
+    {
+        if (x < 0 || (x > 0 && x % 10 == 0))
+        {
+            return false;
+        }
+        int y = 0;
+        for (; y < x; x /= 10)
+        {
+            y = y * 10 + x % 10;
+        }
+        return x == y || x == y / 10;
+    }
+}