feat: add cs solution to lc problem: No.1930 (doocs#1964)

* feat: add cs solution to lc problem: No.1930

* Update README_EN.md to lc problem: No.1930

* Update README.md to lc problem: No.1930

Author: dev-mauli

solution/1900-1999/1930.Unique Length-3 Palindromic Subsequences/README.md

@@ -144,6 +144,25 @@ func countPalindromicSubsequence(s string) (ans int) {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int CountPalindromicSubsequence(string s) {
+        int ans = 0;
+        for (char c = 'a'; c <= 'z'; ++c) {
+            int l = s.IndexOf(c), r = s.LastIndexOf(c);
+            HashSet<char> cs = new HashSet<char>();
+            for (int i = l + 1; i < r; ++i) {
+                cs.Add(s[i]);
+            }
+            ans += cs.Count;
+        }
+        return ans;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1900-1999/1930.Unique Length-3 Palindromic Subsequences/README_EN.md

@@ -126,6 +126,25 @@ func countPalindromicSubsequence(s string) (ans int) {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public int CountPalindromicSubsequence(string s) {
+        int ans = 0;
+        for (char c = 'a'; c <= 'z'; ++c) {
+            int l = s.IndexOf(c), r = s.LastIndexOf(c);
+            HashSet<char> cs = new HashSet<char>();
+            for (int i = l + 1; i < r; ++i) {
+                cs.Add(s[i]);
+            }
+            ans += cs.Count;
+        }
+        return ans;
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1900-1999/1930.Unique Length-3 Palindromic Subsequences/Solution.cs

@@ -0,0 +1,14 @@
+public class Solution {
+    public int CountPalindromicSubsequence(string s) {
+        int ans = 0;
+        for (char c = 'a'; c <= 'z'; ++c) {
+            int l = s.IndexOf(c), r = s.LastIndexOf(c);
+            HashSet<char> cs = new HashSet<char>();
+            for (int i = l + 1; i < r; ++i) {
+                cs.Add(s[i]);
+            }
+            ans += cs.Count;
+        }
+        return ans;
+    }
+}