feat: add cs solution to lc problem: No.1980 (#1973)

* feat: add cs solution to lc problem: No.1980

* Update README_EN.md to lc problem: No.1980

* Update README.md to lc problem: No.1980

Author: dev-mauli

solution/1900-1999/1980.Find Unique Binary String/README.md

@@ -139,6 +139,25 @@ func findDifferentBinaryString(nums []string) string {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public string FindDifferentBinaryString(string[] nums) {
+        int mask = 0;
+        foreach (var x in nums) {
+            int cnt = x.Count(c => c == '1');
+            mask |= 1 << cnt;
+        }
+        int i = 0;
+        while ((mask >> i & 1) == 1) {
+            i++;
+        }
+        return string.Format("{0}{1}", new string('1', i), new string('0', nums.Length - i));
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1900-1999/1980.Find Unique Binary String/README_EN.md

@@ -120,6 +120,25 @@ func findDifferentBinaryString(nums []string) string {
 }
 ```
 
+### **C#**
+
+```cs
+public class Solution {
+    public string FindDifferentBinaryString(string[] nums) {
+        int mask = 0;
+        foreach (var x in nums) {
+            int cnt = x.Count(c => c == '1');
+            mask |= 1 << cnt;
+        }
+        int i = 0;
+        while ((mask >> i & 1) == 1) {
+            i++;
+        }
+        return string.Format("{0}{1}", new string('1', i), new string('0', nums.Length - i));
+    }
+}
+```
+
 ### **...**
 
 ```

solution/1900-1999/1980.Find Unique Binary String/Solution.cs

@@ -0,0 +1,14 @@
+public class Solution {
+    public string FindDifferentBinaryString(string[] nums) {
+        int mask = 0;
+        foreach (var x in nums) {
+            int cnt = x.Count(c => c == '1');
+            mask |= 1 << cnt;
+        }
+        int i = 0;
+        while ((mask >> i & 1) == 1) {
+            i++;
+        }
+        return string.Format("{0}{1}", new string('1', i), new string('0', nums.Length - i));
+    }
+}