2.给出距离1900年1月1日的天数，求日期

#include <iostream>
#include <assert.h>


//判断是否闰年
bool IsLeapYear(unsigned int year)
{
  if ((year % 4 == 0 && year % 100 != 0) || year % 400 == 0)
  {
    return true;
  }
  else
  {
    return false;
  }
}

//根据一年中的第几天(0起始)，返回月份(1起始)，并且置天数为其在本月的索引(0起始)
unsigned int GetMonthByDay(unsigned int &day, bool leap)
{
  assert(day < (leap ? 366 : 365));
  static unsigned int monthtable[] = { 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
  monthtable[1] = leap ? 29 : 28;

  unsigned int  month = 0;
  unsigned int count = 0;
  unsigned int n = day;

  do
  {
    n = day - count;
    count += monthtable[month++];
  } while (day >= count);
  day = n;
  return month;
}

//距离1900年1月1日的天数(0起始)，返回年，并且置天数为其在本年的索引(0起始)
unsigned int GetYearByDay(unsigned int &day)
{
  unsigned int Y = day / 365;
  int D = day % 365;
  int  YY = Y - 1;
  D -= YY / 4;
  D += YY / 100;
  D -= (YY + 300) / 400;
  while (D < 0)
  {
    Y--;
    D += IsLeapYear(Y + 1900) ? 366 : 365;
  }
  day = D;
  return Y + 1900;
}

//距离1900年1月1日的天数(0起始)，计算这一天的日期(年，月，日)(1起始)。
void foo(unsigned int &day, unsigned int &year, unsigned int &month)
{
  year = GetYearByDay(day);
  month = GetMonthByDay(day, IsLeapYear(year));
  day += 1;
}

int main()
{
  unsigned int day = 42696;
  unsigned int year = 0;
  unsigned int month = 0;
  foo(day, year, month);
  std::cout << year << "/" << month << "/" << day << std::endl;
  getchar();
  return 0;
}