/* Multiply Two Long Numbers
------------------------------------
Implements manual big‑integer multiplication
using decimal strings.
*/
using System;
using System.Text;
public sealed class LongNumber
{
public string Value { get; }
public LongNumber(string v) {
Value = Normalize(v);
}
/* Multiply two integer strings a * b and return the result as a string.
Caller receives a normal C# string.
No aliasing issues because strings are immutable. */
public LongNumber Multiply(LongNumber other)
{
string a = Value;
string b = other.Value;
// Handle sign
bool sign = false;
if (a.StartsWith("-")) {
sign = !sign;
a = a[1..];
}
if (b.StartsWith("-")) {
sign = !sign;
b = b[1..];
}
// If either is "0", return "0"
if (a == "0" || b == "0")
return new LongNumber("0");
int la = a.Length;
int lb = b.Length;
int[] digits = new int[la + lb];
/* Multiply from right to left */
for (int i = la - 1; i >= 0; i--) {
char ca = a[i];
if (!char.IsDigit(ca)) continue;
for (int j = lb - 1; j >= 0; j--) {
char cb = b[j];
if (!char.IsDigit(cb)) continue;
int pos = i + j + 1;
int sum = digits[pos] + (ca - '0') * (cb - '0');
digits[pos] = sum % 10;
digits[pos - 1] += sum / 10;
}
}
// Convert digits[] to string, skipping leading zero
var sb = new StringBuilder();
int start = digits[0] == 0 ? 1 : 0;
for (int i = start; i < digits.Length; i++)
sb.Append((char)('0' + digits[i]));
// Add sign if needed
if (sign)
sb.Insert(0, '-');
return new LongNumber(sb.ToString());
}
private static string Normalize(string s)
{
// Skip leading spaces
s = s.TrimStart();
// Remove leading zeros (but leave one zero)
int i = 0;
while (i < s.Length - 1 && s[i] == '0')
i++;
return s[i..];
}
public override string ToString() => Value;
}
public static class Program
{
public static void Main()
{
var a = new LongNumber(" 18361891827367132321");
var b = new LongNumber("-18361891827367132321");
var result = a.Multiply(b);
Console.WriteLine(result);
}
}
/*
run:
-337159071479931885857929667075122847041
*/
