ctci-balanced-brackets.cs

using System;
using System.IO;
using System.Text;

class Solution {
    
    static void Main(String[] args) {
        var t = Convert.ToInt32(Console.ReadLine());
        for(var a0 = 0; a0 < t; a0++) {
            var expression = Console.ReadLine();
            Console.WriteLine(IsBalanced(expression) ? "YES" : "NO");
        }
    }
    
    static bool IsBalanced(string expression) {
        var sb = new StringBuilder(40);
        foreach (var c in expression) {
            if (IsClosingBracket(c)) {
                if (sb.Length == 0) return false;
                if (!IsClosingBracketFor(c, sb[sb.Length-1])) return false;
                sb.Remove(sb.Length-1, 1);
            } else {
                sb.Append(c);
            }
        }
        
        return sb.Length == 0;
    }
    
    static bool IsClosingBracket(char c) {
        return c == '}' || c == ']' || c == ')';
    }
    
    static bool IsClosingBracketFor(char c, char other) {
        return (c == '}' && other == '{')
            || (c == ']' && other == '[')
            || (c == ')' && other == '(');
    }
}

Could possibly extend on the char type and give it the IsClosingBracket and IsClosingBracketFor members.

using System;
class TestCharBracketExtensions {
  public static void Main (string[] args) {
    Console.WriteLine('}'.IsClosingBracket());       // => true
    Console.WriteLine(')'.IsClosingBracketFor('(')); // => true
  }
}

public static class CharBracketExtensions {
  public static bool IsClosingBracket(this char c) {
    return c == '}' || c == ']' || c == ')';
  }
  
  public static bool IsClosingBracketFor(this char c, char other) {
    return (c == '}' && other == '{')
        || (c == ']' && other == '[')
        || (c == ')' && other == '(');
  }
}