num.cc

#include <iostream>
#include <algorithm>
#include <vector>
#include <unordered_set>

static std::vector<int> getDigits(int num)
{
  std::vector<int> digits;

  while (num) {
    digits.push_back(num % 10);
    num = num / 10;
  }

  return digits;
}

static int getNum(const std::vector<int>& digits)
{
  int num = 0;
  int base = 1;

  for (int digit : digits) {
    num += digit * base;
    base *= 10;
  }

  return num;
}

int getGreatestNumRearrange(int num)
{
  std::vector<int> digits = getDigits(num);

  std::sort(digits.begin(), digits.end());

  return getNum(digits);
}

int getNextGreaterNumRearrange(int num)
{
  std::vector<int> digits = getDigits(num);
  int sz = digits.size();

  int i = 0;

  while (i < sz - 1) {
    if (digits[i] > digits[i + 1]) {
      break;
    }
    i++;
  }

  for (int& digit : digits) {
    if (digit > digits[i + 1]) {
      std::swap(digits[i + 1], digit);
      std::sort(digits.begin(), digits.begin() + i + 1, std::greater<int>());
      return getNum(digits);
    }
  }

  return num;
}

/* Greatest number but less than given number */
int getGreatestLessNumRearrange(int num)
{
  std::vector<int> digits = getDigits(num);
  int sz = digits.size();

  for (int i = 0; i < sz - 1; i++) {
    if (digits[i] < digits[i + 1]) {
      std::swap(digits[i], digits[i + 1]);
      std::sort(digits.begin(), digits.begin() + i + 1);
      break;
    }
  }

  return getNum(digits);
}

static std::string printChunk(int num)
{
  std::string str;

  std::string digits_str[] = {"",        "one",     "two",       "three",    "four",
                              "five",    "six",     "seven",     "eight",    "nine",
                              "ten",     "eleven",  "twelve",    "thirteen", "fourteen",
                              "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"};

  std::string tens_str[] = {
      "", "", "twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"};

  if (num < 20) {
    str = digits_str[num];
  } else if (num < 100) {
    str = tens_str[num / 10];
    num = num % 10;
    if (num) {
      str += " ";
      str += digits_str[num];
    }
  } else {
    str = digits_str[num / 100] + " hundred";
    num = num % 100;
    if (num) {
      str += " ";
      str += printChunk(num);
    }
  }

  return str;
}

std::string printNum(int num)
{
  if (num == 0) {
    return "zero";
  }

  std::string str;

  if (num < 0) {
    num = -num;
    str = "negative ";
  }

  std::vector<int> chunk_num;

  while (num) {
    chunk_num.push_back(num % 1000);
    num = num / 1000;
  }

  std::string higher_str[] = {"", " thousand ", " million ", " billion "};

  for (int i = chunk_num.size() - 1; i >= 0; i--) {
    str += printChunk(chunk_num[i]);
    str += higher_str[i];
  }

  return str;
}

bool isPrime(int num)
{
  if (num <= 1) {
    return false;
  }
  if (num <= 3) {
    return true;
  }
  if ((num % 2 == 0) || (num % 3 == 0)) {
    return false;
  }
  for (int i = 5; i * i <= num; i += 6) {
    if ((num % i == 0) || (num % (i + 2) == 0)) {
      return false;
    }
  }
  return true;
}

int getSumOfSquareOfDigits(int num)
{
  int sum = 0;
  while (num) {
    sum += (num % 10) * (num % 10);
    num = num / 10;
  }
  return sum;
}

bool isHappy(int num)
{
  int slow = num;
  int fast = num;

  do {
    slow = getSumOfSquareOfDigits(slow);
    fast = getSumOfSquareOfDigits(fast);
    fast = getSumOfSquareOfDigits(fast);
  } while (slow != fast);

  return slow == 1;
}

int main()
{
  for (int i = 10; i < 100; i++) {
    if (isPrime(i)) {
      std::cout << i << std::endl;
    }
  }
}