public static StringNum Multiply(StringNum a, int b)
{
string result = "";
int aLength = a.reversedDigits.Length;
int carry = 0;
for (int digit = 0; digit <= aLength; digit++)
{
int aN = 0;
if (digit < aLength)
{
aN = a.reversedDigits[digit] - '0';
}
int value = aN * b + carry;
carry = value / 10;
value = value % 10;
result += (char)('0' + value);
}
while (carry != 0)
{
int value = carry % 10;
carry = carry / 10;
result += (char)('0' + value);
}
result = result.TrimEnd('0');
string nonReversed = new string(result.Reverse().ToArray());
return(new StringNum(nonReversed));
}
private string GetAnswerForGivenStrings(string[] longNums)
{
List <StringNum> stringNums = longNums.Select(n => new StringNum(n)).ToList();
StringNum result = stringNums.Aggregate((x, y) => StringNum.Add(x, y));
return(result.ToString().Substring(0, 10));
}
public static StringNum Add(StringNum a, StringNum b)
{
string result = "";
int aLength = a.reversedDigits.Length;
int bLength = b.reversedDigits.Length;
int maxLen = aLength;
if (bLength > maxLen)
{
maxLen = bLength;
}
maxLen++;
int carry = 0;
for (int digit = 0; digit < maxLen; digit++)
{
int aN = 0;
if (digit < aLength)
{
aN = a.reversedDigits[digit] - '0';
}
int bN = 0;
if (digit < bLength)
{
bN = b.reversedDigits[digit] - '0';
}
int value = aN + bN + carry;
carry = 0;
while (value >= 10)
{
carry++;
value -= 10;
}
result += (char)('0' + value);
}
result = result.TrimEnd('0');
string nonReversed = new string(result.Reverse().ToArray());
return(new StringNum(nonReversed));
}
public override string GetAnswer()
{
// yet another BigInteger=Cheating problem.
// (then again, at this point, I'm reusing the same library I wrote
// so I'm not sure there's even a point to it...)
StringNum a = new StringNum("1");
StringNum b = new StringNum("1");
StringNum c = new StringNum("2");
int cIndex = 3;
while (c.ToString().Length < 1000)
{
a = b;
b = c;
c = StringNum.Add(a, b);
cIndex++;
}
return(cIndex.ToString());
}
private int GetAnswerForFactorial(int factorial)
{
StringNum num = new StringNum("1");
for (int i = 2; i <= factorial; i++)
{
num = StringNum.Multiply(num, i);
}
string finalFactorial = num.ToString();
int answer = 0;
foreach (char c in finalFactorial)
{
answer += (c - '0');
}
return(answer);
}
public override string GetAnswer()
{
List <string> possibilities = new List <string>();
Func <string, char, string> AppendNewChar = (s, c) => s + c;
Func <string, char, string> PrefixNewChar = (s, c) => c + s;
possibilities.Add("");
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 2);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 3);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 5);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 7);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 11);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 13);
possibilities = ApplyEachPossibleDigit(possibilities, AppendNewChar);
possibilities = RemoveEntriesWhereLastThreeDontDivideBy(possibilities, 17);
possibilities = ApplyEachPossibleDigit(possibilities, PrefixNewChar);
StringNum answer = possibilities.Select(s => new StringNum(s)).Aggregate((x, y) => StringNum.Add(x, y));
return(answer.ToString());
}
private long GetAnswerForSpecificPower(int powerOfTwo)
{
// I couldn't figure out any mathematical trick to derive this other than
// simply multiplying out the number 2^1000 and then checking.
// Also, once again, using BigInteger seemed way too much like cheating
// (pretty sure these problems were written before the BigInteger library was available?)
StringNum myVal = new StringNum("2");
for (int i = 2; i <= powerOfTwo; i++)
{
myVal = StringNum.Add(myVal, myVal);
}
string result = myVal.ToString();
int charSum = 0;
foreach (char c in result)
{
charSum += (c - '0');
}
return(charSum);
}