Your code failed for cases when your string consists of characters like ( and )

Please, help me to understand. My C# code has passed all basic tests but has failed Parentheses Tests and Random Tests.
ParenthesesTests
Test Failed
Expected string length 6 but was 5. Strings differ at index 0.
Expected: ")))))("
But was: "((((("
-----------^

RandomTests
Test Failed
String lengths are both 18. Strings differ at index 3.
Expected: "((()())(()()()(((("
But was: "(((((((((((((((((("
--------------^

Use System.Linq in the using section. It helps