foo.cc: In function 'int passDays(int, int, int)':
foo.cc:36:9: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
   36 |  return sum;
      |         ^~~
foo.cc: In function 'int weekdayOfTheFirstDay(int, int)':
foo.cc:51:35: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
   51 |  days += passDays(year, month, 1) - 1 ;//减掉1800年的1号
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~^~~

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