foo.cc: In function 'int main()':
foo.cc:17:2: error: expected ';' before 'for'
  for(int i=1;i<=n;i++)
  ^~~
foo.cc:17:14: error: 'i' was not declared in this scope
  for(int i=1;i<=n;i++)
              ^
foo.cc:19:4: error: 'else' without a previous 'if'
    else b[n+1-i/2]=a[i];//b数组为排完序的数组 
    ^~~~

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