用c#编写一个函数，列出一个字符串列表，此字符串表示平衡n对括号的所有方法的排列组合。
示例：
BalancedParens(0) returns List<string> with element: ""
BalancedParens(1) returns List<string> with element: "()"
BalancedParens(2) returns List<string> with elements: "()()","(())"
BalancedParens(3) returns List<string> with elements: "()()()","(())()","()(())","(()())","((()))"