foo.cc: In function 'void Build()':
foo.cc:54:2: warning: operation on 'j' may be undefined [-Wsequence-point]
 j=j-1<0?0:--j;
 ~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:73:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&n);
 ~~~~~^~~~~~~~~

[Hydro](https://hydro.ac)提供评测服务