text: #include "BigNum.h"
  #include "StrUtil.h"
  #include <iostream>
  
  namespace {
      bool checkNum(const std::string& s)
      {
          for (char c : s)
          {
              c = c - '0';
              if (c > 9 || c < 0)  return false;
          }
          return true;
      }
  }
  
  
  std::string multBigNum(const std::string& first, const std::string& second)
  {
      std::string result = "0";
  
      bool b = checkNum(first);
      if (!b) return INVALID_NUM;
  
      b = checkNum(second);
      if (!b) return INVALID_NUM;
  
      for (int i = second.length() - 1; i >= 0; i--)
      {
          std::string s = multStrAndChar(first, second[i]);
          s = s.insert(s.length() - 1, second.length() - 1 - i, '0');
          result = addStr(result, s);
      }
      return result;
  }
  
  