foo.cc: In function 'int IsLeapYear(int)':
foo.cc:5:27: error: expected primary-expression before '||' token
    5 |   if(year%4==0&&year%100!=||year%400==0) return 1;
      |                           ^~
foo.cc:7:1: warning: control reaches end of non-void function [-Wreturn-type]
    7 | }
      | ^

[Hydro](https://hydro.ac)提供评测服务