public class Solution
    {
        string ConverToHex(string bit)
        {
            var s = "";
            switch (bit)
            {
                case "0000":
                    s = "0";
                    break;
                case "0001":
                    s = "1";
                    break;
                case "0010":
                    s = "2";
                    break;
                case "0011":
                    s = "3";
                    break;
                case "0100":
                    s = "4";
                    break;
                case "0101":
                    s = "5";
                    break;
                case "0110":
                    s = "6";
                    break;
                case "0111":
                    s = "7";
                    break;
                case "1000":
                    s = "8";
                    break;
                case "1001":
                    s = "9";
                    break;
                case "1010":
                    s = "a";
                    break;
                case "1011":
                    s = "b";
                    break;
                case "1100":
                    s = "c";
                    break;
                case "1101":
                    s = "d";
                    break;
                case "1110":
                    s = "e";
                    break;
                case "1111":
                    s = "f";
                    break;
            }
            return s;
        }

        public string ToHex(int num)
        {
            var fuhao = 1;
            if (num < 0)
            {
                fuhao = -1;
                num = num * (-1);
            }

            //10进制转16进制，负数使用补码
            var bitAry = new string[32];
            for (int i = 0; i < bitAry.Length; i++)
            {
                bitAry[i] = "0";
            }
            //STEP1:十进制转二进制
            int index = 0;
            do
            {
                var x = num % 2;
                bitAry[index++] = x.ToString();
                num = num / 2;
            } while (num != 0);

            var list = bitAry.ToList();

            //STEP2:正数使用原码，负数转补码
            if (fuhao == -1)
            {
                list.Clear();

                bitAry[bitAry.Length - 1] = "1";//符号为设置为0

                //计算反码
                StringBuilder sbFan = new StringBuilder();
                for (int i = 0; i < bitAry.Length - 1; i++)
                {
                    if (bitAry[i] == "1")
                    {
                        sbFan.Append("0");
                    }
                    else
                    {
                        sbFan.Append("1");
                    }
                }
                sbFan.Append("1");//index=31的是符号位

                //反码加1

                var fanma = sbFan.ToString();

                var step = 0;
                var first = true;
                for (int i = 0; i < fanma.Length - 1; i++)
                {
                    var x = Convert.ToInt32(fanma[i].ToString());
                    if (first)
                    {
                        first = false;
                        x = (x + 1) % 2;
                        list.Add(x.ToString());
                        if (x == 0)
                        {
                            step = 1;
                        }
                        else
                        {
                            step = 0;
                        }
                    }
                    else
                    {
                        list.Add(((x + step) % 2).ToString());
                        if (x == 1 && step == 1)
                        {
                            step = 1;
                        }
                        else
                        {
                            step = 0;
                        }
                    }
                }
                list.Add("1");
            }


            //STEP3:二进制转十六进制
            string result = "";

            if (fuhao == -1)//负数
            {
                var temp = "";
                for (int i = 0; i < list.Count; i++)
                {

                    if ((i + 1) % 4 != 0)//0,1,2
                    {
                        temp = list[i] + temp;
                    }
                    else
                    {
                        temp = list[i] + temp;
                        var hex = ConverToHex(temp);
                        result = hex + result;
                        temp = "";
                    }
                }
            }
            else//正数
            {
                var temp = "";
                for (int i = 0; i < list.Count; i++)
                {
                    if ((i + 1) % 4 != 0)
                    {
                        temp = list[i] + temp;
                    }
                    else
                    {
                        temp = list[i] + temp;
                        var hex = ConverToHex(temp);
                        result = hex + result;
                        temp = "";
                    }

                }

            }

            var notZeroIndex = -1;
            for (int i = 0; i < result.Length; i++)
            {
                if (result[i] != ‘0‘)
                {
                    notZeroIndex = i;
                    break;
                }
            }
            if (notZeroIndex > 0)
            {
                result = result.Substring(notZeroIndex);
            }

            if (result == "00000000")
            {
                result = "0";
            }

            return result;
        }
    }