MS Codility Test

one.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace interview {
    class Program {

        // check if the last 2 chars with c are the same 
        public static bool predicate (string s, char c) {
            if (s.Length < 2) {
                return true;
            } else {
                var l = new List<char> {s[s.Length - 1],s[s.Length -2]};
                return !l.TrueForAll (elem => elem == c);
            }
        }

        public static string  append (string s, char c) {
            var sl = s.ToList ();
            sl.Add (c);
            return new String (sl.ToArray ());
        }
        
        public static List<string> helper (string prev, int A, int B, int C) {
            // 1. we can generate all possible solutions 
            // 2. or we can backtrack 

            // three cases: pick A, B or C (iff not zero, obviously)
            var table = new Dictionary<char, int> {
                {'A', A}, 
                {'B', B}, 
                {'C', C}
            };
          
            var al = new List<string> ();
            var bl = new List<string> ();
            var cl = new List<string> ();
            // This is just a fold 
            foreach (var pair in table) {
                var c = pair.Key;
                var n = pair.Value;
                if (n > 0) {
                    if (c == 'A') {
                        if (predicate (prev, char.ToLower (c))) {
                            var a = append (prev, 'a');
                            al = helper (a, A-1,B,C);
                            // sol.Concat (s);
                        }
                    } else if (c == 'B') {
                         if (predicate (prev, char.ToLower (c))) {
                            var a = append (prev, 'b');
                            bl = helper (a, A,B-1,C);
                            // sol.Concat (s);
                        }
                    } else if (c == 'C') {
                        if (predicate (prev, char.ToLower (c))) {
                            var a = append (prev, 'c');
                            cl = helper (a, A,B,C-1);
                            // sol.Concat (s);
                        }
                    }
                }
            }

            var sol = al.Concat (bl).Concat (cl).ToList ();
            if (sol.Count == 0) {
                sol.Add (prev);
            }
            
            return sol;
        } 

       public static string solution (int A, int B, int C) {
           // Generate list of all possible solutions
            var l = helper ("", A, B, C);
            if (l.Count == 0) {
                return "";
            } else { 
                return l[0];
            }
        }
        static void Main(string[] args) {
            Console.WriteLine (solution (7, 3, 3));
        }
    }
}

three.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace interview {
    class Program {

        public static char Flip (char c) {
            if (char.IsUpper (c)) {
                return char.ToLower (c);
            } else {
                return char.ToUpper (c);
            }
        }
        public static string solution(string S) {
            // create a maping form letters to numbers 
            var seen = new List<char> ();
            // create a list of letters 
            var ds = new List<char> ();

            // once we find second put in a list 

            foreach (var c in S) {
               if (seen.Contains (Flip (c)) ) {
                   ds.Add (char.ToUpper (c));
                   // update seen 
                   seen.Add (c);
               } else {
                   seen.Add (c);
               }
            }

            // find the minimun from the list 
            if (ds.Count == 0) {
                return "NO";
            } else {
                return ds.Max ().ToString ();
            }
        }

        static void Main(string[] args) {
            
            Console.WriteLine("");
        }
    }
}

two.cs

using System;
using System.Collections.Generic;

namespace interview {
    class Program {
         public int[] solution(int N) {
            // two cases
            // n is even
            // n is odd 
            var sol = new List<int> ();
            if (N % 2 == 0) {
                // even
                var count = 1;
                while (N > 0) {
                    sol.Add (count);
                    sol.Add (-count);
                    count++;
                    N = N-2;
                }
            } else {
                // odd
                var count = 1;
                sol.Add (0);
                N = N - 1;
                while (N > 0) {
                    sol.Add (count);
                    sol.Add (-count);
                    count++;
                    N = N-2;
                }
            }

            return sol.ToArray ();
        }

        static void Main(string[] args) {
            
            Console.WriteLine("");
        }
    }
}