cs: add «678. Valid Parenthesis String»

Signed-off-by: Matej Focko <mfocko@redhat.com>

cs/valid-parenthesis-string.cs

@@ -0,0 +1,26 @@
+public class Solution {
+    public bool CheckValidString(string s) {
+        int left = 0;
+        int right = 0;
+
+        for (int i = 0, j = s.Length - 1; i < s.Length; ++i, --j) {
+            if (s[i] == '(' || s[i] == '*') {
+                ++left;
+            } else {
+                --left;
+            }
+
+            if (s[j] == ')' || s[j] == '*') {
+                ++right;
+            } else {
+                --right;
+            }
+
+            if (left < 0 || right < 0) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}