bijectiveEncodeDecode.cs

        [Test]
        public void decodeTest() {
            Assert.AreEqual(19158, decode("e9a")); 
        }

        [Test]
        public void encodeTest()
        {
            Assert.AreEqual("cb", encode(125));
        }

        private String ALPHABET = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        private int BASE = 62;

        public string encode(int num)
        {
            StringBuilder sb = new StringBuilder();

            while ( num > 0 )
            {
                sb.Append( ALPHABET[num % BASE]);
                num /= BASE;
            }

            var items = sb.ToString().ToCharArray();
            Array.Reverse(items);
            return new string(items);   
        }

        public int decode(string str)
        {
            int num = 0;

            for ( int i = 0, len = str.Length; i < len; i++ )
            {
                num = num * BASE + ALPHABET.IndexOf(str[i]); 
            }

            return num;
        }