feat: add csharp solution to lc problem: No.0020 (doocs#1931)

Author: dev-mauli

solution/0000-0099/0020.Valid Parentheses/solutions.cs

@@ -0,0 +1,17 @@
+public class Solution {
+    public bool IsValid(string s) {
+        Stack<char> stk = new Stack<char>();
+        foreach (var c in s.ToCharArray()) {
+            if (c == '(') {
+                stk.Push(')');
+            } else if (c == '[') {
+                stk.Push(']');
+            } else if (c == '{') {
+                stk.Push('}');
+            } else if (stk.Count == 0 || stk.Pop() != c) {
+                return false;
+            }
+        }
+        return stk.Count == 0;
+    }
+}