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();
            }
        }
    }
}

C++: #include <iomanip>
#include <iostream>
#include <tuple>
#include <vector>

const std::string DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

std::string encodeNegativeBase(int64_t n, int b) {
    if (b < -62 || b > -1) {
        throw std::runtime_error("Argument out of range: b");
    }
    if (n == 0) {
        return "0";
    }

    std::string output;
    int64_t nn = n;
    while (nn != 0) {
        int rem = nn % b;
        nn /= b;
        if (rem < 0) {
            nn++;
            rem -= b;
        }
        output += DIGITS[rem];
    }

    std::reverse(output.begin(), output.end());
    return output;
}

int64_t decodeNegativeBase(const std::string& ns, int b) {
    if (b < -62 || b > -1) {
        throw std::runtime_error("Argument out of range: b");
    }
    if (ns == "0") {
        return 0;
    }

    int64_t total = 0;
    int64_t bb = 1;

    for (auto it = ns.crbegin(); it != ns.crend(); it = std::next(it)) {
        auto ptr = std::find(DIGITS.cbegin(), DIGITS.cend(), *it);
        if (ptr != DIGITS.cend()) {
            auto idx = ptr - DIGITS.cbegin();
            total += idx * bb;
        }
        bb *= b;
    }
    return total;
}

int main() {
    using namespace std;

    vector<pair<int64_t, int>> nbl({
        make_pair(10, -2),
        make_pair(146, -3),
        make_pair(15, -10),
        make_pair(142961, -62)
        });

    for (auto& p : nbl) {
        string ns = encodeNegativeBase(p.first, p.second);
        cout << setw(12) << p.first << " encoded in base " << setw(3) << p.second << " = " << ns.c_str() << endl;

        int64_t n = decodeNegativeBase(ns, p.second);
        cout << setw(12) << ns.c_str() << " decoded in base " << setw(3) << p.second << " = " << n << endl;

        cout << endl;
    }

    return 0;
}