foo.cc:29:2: error: #endif without #if
 #endif
  ^~~~~
foo.cc:30:0: error: unterminated #ifdef
 #ifdef method_3
 
foo.cc: In function 'int main()':
foo.cc:23:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&n);
     ~~~~~^~~~~~~~~

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