Task: Negative base numbers
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;
}

Java: import java.util.List;
import java.util.Map;
import java.util.Objects;

public class NegativeBaseNumbers {
    private static final String DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";

    private static String encodeNegBase(long n, int b) {
        if (b < -62 || b > -1) throw new IllegalArgumentException("Parameter b is out of bounds");
        if (n == 0) return "0";
        StringBuilder out = new StringBuilder();
        long nn = n;
        while (nn != 0) {
            int rem = (int) (nn % b);
            nn /= b;
            if (rem < 0) {
                nn++;
                rem -= b;
            }
            out.append(DIGITS.charAt(rem));
        }
        out.reverse();
        return out.toString();
    }

    private static long decodeNegBase(String ns, int b) {
        if (b < -62 || b > -1) throw new IllegalArgumentException("Parameter b is out of bounds");
        if (Objects.equals(ns, "0")) return 0;
        long total = 0;
        long bb = 1;
        for (int i = ns.length() - 1; i >= 0; i--) {
            char c = ns.charAt(i);
            total += DIGITS.indexOf(c) * bb;
            bb *= b;
        }
        return total;
    }

    public static void main(String[] args) {
        List<Map.Entry<Long, Integer>> nbl = List.of(
                Map.entry(10L, -2),
                Map.entry(146L, -3),
                Map.entry(15L, -10),
                Map.entry(-4393346L, -62)
        );
        for (Map.Entry<Long, Integer> p : nbl) {
            String ns = encodeNegBase(p.getKey(), p.getValue());
            System.out.printf("%12d encoded in base %-3d = %s\n", p.getKey(), p.getValue(), ns);
            long n = decodeNegBase(ns, p.getValue());
            System.out.printf("%12s decoded in base %-3d = %d\n\n", ns, p.getValue(), n);
        }
    }
}