Task: Negative base numbers
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace NegativeBaseNumbers {
    class Program {
        const string DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

        static string EncodeNegativeBase(long n, int b) {
            if (b < -62 || b > -1) {
                throw new ArgumentOutOfRangeException("b");
            }
            if (n == 0) {
                return "0";
            }
            StringBuilder output = new StringBuilder();
            long nn = n;
            while (nn != 0) {
                int rem = (int)(nn % b);
                nn /= b;
                if (rem < 0) {
                    nn++;
                    rem -= b;
                }
                output.Append(DIGITS[rem]);
            }
            return new string(output.ToString().Reverse().ToArray());
        }

        static long DecodeNegativeBase(string ns, int b) {
            if (b < -62 || b > -1) {
                throw new ArgumentOutOfRangeException("b");
            }
            if (ns == "0") {
                return 0;
            }
            long total = 0;
            long bb = 1;
            for (int i = ns.Length - 1; i >= 0; i--) {
                char c = ns[i];
                total += DIGITS.IndexOf(c) * bb;
                bb *= b;
            }
            return total;
        }

        static void Main(string[] args) {
            List<Tuple<long, int>> nbl = new List<Tuple<long, int>>() {
                new Tuple<long, int>(10,-2),
                new Tuple<long, int>(146,-3),
                new Tuple<long, int>(15,-10),
                new Tuple<long, int>(-34025238427,-62),
            };
            foreach (var p in nbl) {
                string ns = EncodeNegativeBase(p.Item1, p.Item2);
                Console.WriteLine("{0,12} encoded in base {1,-3} = {2}", p.Item1, p.Item2, ns);
                long n = DecodeNegativeBase(ns, p.Item2);
                Console.WriteLine("{0,12} decoded in base {1,-3} = {2}", ns, p.Item2, n);
                Console.WriteLine();
            }
        }
    }
}

Python: 
from __future__ import print_function

def EncodeNegBase(n, b): 
	if n == 0:
		return "0"
	out = []
	while n != 0:
		n, rem = divmod(n, b)
		if rem < 0:
			n += 1
			rem -= b
		out.append(rem)
	return "".join(map(str, out[::-1]))

def DecodeNegBase(nstr, b): 
	if nstr == "0":
		return 0
	
	total = 0
	for i, ch in enumerate(nstr[::-1]):
		total += int(ch) * b**i
	return total

if __name__=="__main__":
	
	print ("Encode 10 as negabinary (expect 11110)")
	result = EncodeNegBase(10, -2)
	print (result)
	if DecodeNegBase(result, -2) == 10: print ("Converted back to decimal")
	else: print ("Error converting back to decimal")

	print ("Encode 146 as negaternary (expect 21102)")
	result = EncodeNegBase(146, -3)
	print (result)
	if DecodeNegBase(result, -3) == 146: print ("Converted back to decimal")
	else: print ("Error converting back to decimal")

	print ("Encode 15 as negadecimal (expect 195)")
	result = EncodeNegBase(15, -10)
	print (result)
	if DecodeNegBase(result, -10) == 15: print ("Converted back to decimal")
	else: print ("Error converting back to decimal")