foo.cc: In function 'void Build()':
foo.cc:41:2: warning: operation on 'j' may be undefined [-Wsequence-point]
   41 | j=j-1<0?0:--j;
      | ~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:60:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   60 | scanf("%d",&n);
      | ~~~~~^~~~~~~~~

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