Task: Balanced ternary
C: #include <stdio.h>
#include <string.h>

void reverse(char *p) {
    size_t len = strlen(p);
    char *r = p + len - 1;
    while (p < r) {
        *p ^= *r;
        *r ^= *p;
        *p++ ^= *r--;
    }
}

void to_bt(int n, char *b) {
    static char d[] = { '0', '+', '-' };
    static int v[] = { 0, 1, -1 };

    char *ptr = b;
    *ptr = 0;

    while (n) {
        int r = n % 3;
        if (r < 0) {
            r += 3;
        }

        *ptr = d[r];
        *(++ptr) = 0;

        n -= v[r];
        n /= 3;
    }

    reverse(b);
}

int from_bt(const char *a) {
    int n = 0;

    while (*a != '\0') {
        n *= 3;
        if (*a == '+') {
            n++;
        } else if (*a == '-') {
            n--;
        }
        a++;
    }

    return n;
}

char last_char(char *ptr) {
    char c;

    if (ptr == NULL || *ptr == '\0') {
        return '\0';
    }

    while (*ptr != '\0') {
        ptr++;
    }
    ptr--;

    c = *ptr;
    *ptr = 0;
    return c;
}

void add(const char *b1, const char *b2, char *out) {
    if (*b1 != '\0' && *b2 != '\0') {
        char c1[16];
        char c2[16];
        char ob1[16];
        char ob2[16];
        char d[3] = { 0, 0, 0 };
        char L1, L2;

        strcpy(c1, b1);
        strcpy(c2, b2);
        L1 = last_char(c1);
        L2 = last_char(c2);
        if (L2 < L1) {
            L2 ^= L1;
            L1 ^= L2;
            L2 ^= L1;
        }

        if (L1 == '-') {
            if (L2 == '0') {
                d[0] = '-';
            }
            if (L2 == '-') {
                d[0] = '+';
                d[1] = '-';
            }
        }
        if (L1 == '+') {
            if (L2 == '0') {
                d[0] = '+';
            }
            if (L2 == '-') {
                d[0] = '0';
            }
            if (L2 == '+') {
                d[0] = '-';
                d[1] = '+';
            }
        }
        if (L1 == '0') {
            if (L2 == '0') {
                d[0] = '0';
            }
        }

        add(c1, &d[1], ob1);
        add(ob1, c2, ob2);
        strcpy(out, ob2);

        d[1] = 0;
        strcat(out, d);
    } else if (*b1 != '\0') {
        strcpy(out, b1);
    } else if (*b2 != '\0') {
        strcpy(out, b2);
    } else {
        *out = '\0';
    }
}

void unary_minus(const char *b, char *out) {
    while (*b != '\0') {
        if (*b == '-') {
            *out++ = '+';
            b++;
        } else if (*b == '+') {
            *out++ = '-';
            b++;
        } else {
            *out++ = *b++;
        }
    }
    *out = '\0';
}

void subtract(const char *b1, const char *b2, char *out) {
    char buf[16];
    unary_minus(b2, buf);
    add(b1, buf, out);
}

void mult(const char *b1, const char *b2, char *out) {
    char r[16] = "0";
    char t[16];
    char c1[16];
    char c2[16];
    char *ptr = c2;

    strcpy(c1, b1);
    strcpy(c2, b2);

    reverse(c2);

    while (*ptr != '\0') {
        if (*ptr == '+') {
            add(r, c1, t);
            strcpy(r, t);
        }
        if (*ptr == '-') {
            subtract(r, c1, t);
            strcpy(r, t);
        }
        strcat(c1, "0");

        ptr++;
    }

    ptr = r;
    while (*ptr == '0') {
        ptr++;
    }
    strcpy(out, ptr);
}

int main() {
    const char *a = "+-0++0+";
    char b[16];
    const char *c = "+-++-";
    char t[16];
    char d[16];

    to_bt(-436, b);
    subtract(b, c, t);
    mult(a, t, d);

    printf("      a: %14s %10d\n", a, from_bt(a));
    printf("      b: %14s %10d\n", b, from_bt(b));
    printf("      c: %14s %10d\n", c, from_bt(c));
    printf("a*(b-c): %14s %10d\n", d, from_bt(d));

    return 0;
}

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;
	}
}