public class Solution {    public string ConvertToBase7(int num) {        var prefix = num >= 0 ? "" : "-";            var list = new List
     
      ();            StringBuilder sb = new StringBuilder();            num = Math.Abs(num);            do            {                var n = num % 7;                sb.Append(n.ToString());                num = num / 7;            } while (num != 0);            var result = sb.ToString() + prefix;            var list2 = result.Reverse();            sb.Clear();            foreach (var c in list2)            {                sb.Append(c);            }            return sb.ToString();    }}