Task: Balanced ternary
C#: using System;
using System.Text;
using System.Collections.Generic;

public class BalancedTernary
{
	public static void Main()
	{
		BalancedTernary a = new BalancedTernary("+-0++0+");
		System.Console.WriteLine("a: " + a + " = " + a.ToLong());
		BalancedTernary b = new BalancedTernary(-436);
		System.Console.WriteLine("b: " + b + " = " + b.ToLong());
		BalancedTernary c = new BalancedTernary("+-++-");
		System.Console.WriteLine("c: " + c + " = " + c.ToLong());
		BalancedTernary d = a * (b - c);
		System.Console.WriteLine("a * (b - c): " + d + " = " + d.ToLong());
	}

	private enum BalancedTernaryDigit
	{
		MINUS = -1,
		ZERO = 0,
		PLUS = 1
	}

	private BalancedTernaryDigit[] value;

	
	public BalancedTernary()
	{
		this.value = new BalancedTernaryDigit[0];
	}

	
	public BalancedTernary(String str)
	{
		this.value = new BalancedTernaryDigit[str.Length];
		for (int i = 0; i < str.Length; ++i)
		{
			switch (str[i])
			{
				case '-':
					this.value[i] = BalancedTernaryDigit.MINUS;
					break;
				case '0':
					this.value[i] = BalancedTernaryDigit.ZERO;
					break;
				case '+':
					this.value[i] = BalancedTernaryDigit.PLUS;
					break;
				default:
					throw new ArgumentException("Unknown Digit: " + str[i]);
			}
		}
		Array.Reverse(this.value);
	}

	
	public BalancedTernary(long l)
	{
		List<BalancedTernaryDigit> value = new List<BalancedTernaryDigit>();
		int sign = Math.Sign(l);
		l = Math.Abs(l);
		
		while (l != 0)
		{
			byte rem = (byte)(l % 3);
			switch (rem)
			{
				case 0:
				case 1:
					value.Add((BalancedTernaryDigit)rem);
					l /= 3;
					break;
				case 2:
					value.Add(BalancedTernaryDigit.MINUS);
					l = (l + 1) / 3;
					break;
			}
		}

		this.value = value.ToArray();
		if (sign < 0)
		{
			this.Invert();
		}
	}

	
	public BalancedTernary(BalancedTernary origin)
	{
		this.value = new BalancedTernaryDigit[origin.value.Length];
		Array.Copy(origin.value, this.value, origin.value.Length);
	}

	
	private BalancedTernary(BalancedTernaryDigit[] value)
	{
		int end = value.Length - 1;
		while (value[end] == BalancedTernaryDigit.ZERO)
			--end;
		this.value = new BalancedTernaryDigit[end + 1];
		Array.Copy(value, this.value, end + 1);
	}

	
	private void Invert()
	{
		for (int i=0; i < this.value.Length; ++i)
		{
			this.value[i] = (BalancedTernaryDigit)(-(int)this.value[i]);
		}
	}

	
	override public String ToString()
	{
		StringBuilder result = new StringBuilder();
		for (int i = this.value.Length - 1; i >= 0; --i)
		{
			switch (this.value[i])
			{
				case BalancedTernaryDigit.MINUS:
					result.Append('-');
					break;
				case BalancedTernaryDigit.ZERO:
					result.Append('0');
					break;
				case BalancedTernaryDigit.PLUS:
					result.Append('+');
					break;
			}
		}
		return result.ToString();
	}

	
	public long ToLong()
	{
		long result = 0;
		int digit;
		for (int i = 0; i < this.value.Length; ++i)
		{
			result += (long)this.value[i] * (long)Math.Pow(3.0, (double)i);
		}
		return result;
	}

	
	public static BalancedTernary operator -(BalancedTernary origin)
	{
		BalancedTernary result = new BalancedTernary(origin);
		result.Invert();
		return result;
	}

	
	private static BalancedTernaryDigit carry = BalancedTernaryDigit.ZERO;
	private static BalancedTernaryDigit Add(BalancedTernaryDigit a, BalancedTernaryDigit b)
	{
		if (a != b)
		{
			carry = BalancedTernaryDigit.ZERO;
			return (BalancedTernaryDigit)((int)a + (int)b);
		}
		else
		{
			carry = a;
			return (BalancedTernaryDigit)(-(int)b);
		}
	}

	
	public static BalancedTernary operator +(BalancedTernary a, BalancedTernary b)
	{
		int maxLength = Math.Max(a.value.Length, b.value.Length);
		BalancedTernaryDigit[] resultValue = new BalancedTernaryDigit[maxLength + 1];
		for (int i=0; i < maxLength; ++i)
		{
			if (i < a.value.Length)
			{
				resultValue[i] = Add(resultValue[i], a.value[i]);
				resultValue[i+1] = carry;
			}
			else
			{
				carry = BalancedTernaryDigit.ZERO;
			}
			
			if (i < b.value.Length)
			{
				resultValue[i] = Add(resultValue[i], b.value[i]);
				resultValue[i+1] = Add(resultValue[i+1], carry);
			}
		}
		return new BalancedTernary(resultValue);
	}

	
	public static BalancedTernary operator -(BalancedTernary a, BalancedTernary b)
	{
		return a + (-b);
	}

	
	public static BalancedTernary operator *(BalancedTernary a, BalancedTernary b)
	{
		BalancedTernaryDigit[] longValue = a.value;
		BalancedTernaryDigit[] shortValue = b.value;
		BalancedTernary result = new BalancedTernary();
		if (a.value.Length < b.value.Length)
		{
			longValue = b.value;
			shortValue = a.value;
		}

		for (int i = 0; i < shortValue.Length; ++i)
		{
			if (shortValue[i] != BalancedTernaryDigit.ZERO)
			{
				BalancedTernaryDigit[] temp = new BalancedTernaryDigit[i + longValue.Length];
				for (int j = 0; j < longValue.Length; ++j)
				{
					temp[i+j] = (BalancedTernaryDigit)((int)shortValue[i] * (int)longValue[j]);
				}
				result = result + new BalancedTernary(temp);
			}
		}
		return result;
	}
}

Python: class BalancedTernary:
    

    str2dig = {'+': 1, '-': -1, '0': 0} 
    dig2str = {1: '+', -1: '-', 0: '0'} 
    table = ((0, -1), (1, -1), (-1, 0), (0, 0), (1, 0), (-1, 1), (0, 1)) 

    def __init__(self, inp):
        if isinstance(inp, str):
            self.digits = [BalancedTernary.str2dig[c] for c in reversed(inp)]
        elif isinstance(inp, int):
            self.digits = self._int2ternary(inp)
        elif isinstance(inp, BalancedTernary):
            self.digits = list(inp.digits)
        elif isinstance(inp, list):
            if all(d in (0, 1, -1) for d in inp):
                self.digits = list(inp)
            else:
                raise ValueError("BalancedTernary: Wrong input digits.")
        else:
            raise TypeError("BalancedTernary: Wrong constructor input.")

    @staticmethod
    def _int2ternary(n):
        if n == 0: return []
        if (n % 3) == 0: return [0] + BalancedTernary._int2ternary(n // 3)
        if (n % 3) == 1: return [1] + BalancedTernary._int2ternary(n // 3)
        if (n % 3) == 2: return [-1] + BalancedTernary._int2ternary((n + 1) // 3)

    def to_int(self):
        return reduce(lambda y,x: x + 3 * y, reversed(self.digits), 0)

    def __repr__(self):
        if not self.digits: return "0"
        return "".join(BalancedTernary.dig2str[d] for d in reversed(self.digits))

    @staticmethod
    def _neg(digs):
        return [-d for d in digs]

    def __neg__(self):
        return BalancedTernary(BalancedTernary._neg(self.digits))

    @staticmethod
    def _add(a, b, c=0):
        if not (a and b):
            if c == 0:
                return a or b
            else:
                return BalancedTernary._add([c], a or b)
        else:
            (d, c) = BalancedTernary.table[3 + (a[0] if a else 0) + (b[0] if b else 0) + c]
            res = BalancedTernary._add(a[1:], b[1:], c)
            
            if res or d != 0:
                return [d] + res
            else:
                return res

    def __add__(self, b):
        return BalancedTernary(BalancedTernary._add(self.digits, b.digits))

    def __sub__(self, b):
        return self + (-b)

    @staticmethod
    def _mul(a, b):
        if not (a and b):
            return []
        else:
            if   a[0] == -1: x = BalancedTernary._neg(b)
            elif a[0] ==  0: x = []
            elif a[0] ==  1: x = b
            else: assert False
            y = [0] + BalancedTernary._mul(a[1:], b)
            return BalancedTernary._add(x, y)

    def __mul__(self, b):
        return BalancedTernary(BalancedTernary._mul(self.digits, b.digits))


def main():
    a = BalancedTernary("+-0++0+")
    print "a:", a.to_int(), a

    b = BalancedTernary(-436)
    print "b:", b.to_int(), b

    c = BalancedTernary("+-++-")
    print "c:", c.to_int(), c

    r = a * (b - c)
    print "a * (b - c):", r.to_int(), r

main()