foo.cc: In function 'int main()':
foo.cc:25:7: error: no match for 'operator+=' (operand types are 'std::stack<int>' and '__gnu_cxx::__alloc_traits<std::allocator<char> >::value_type {aka char}')
     fu+=a[i];      
foo.cc:26:29: error: invalid operands of types '<unresolved overloaded function type>' and 'char' to binary 'operator=='
     while(a[i]!='('&&(fu.top=='+'||fu.top=='-'||fu.top=='*'|fu.top=='/'))
                       ~~~~~~^~~~~
foo.cc:26:42: error: invalid operands of types '<unresolved overloaded function type>' and 'char' to binary 'operator=='
     while(a[i]!='('&&(fu.top=='+'||fu.top=='-'||fu.top=='*'|fu.top=='/'))
                                    ~~~~~~^~~~~
foo.cc:26:55: error: invalid operands of types '<unresolved overloaded function type>' and 'char' to binary 'operator=='
     while(a[i]!='('&&(fu.top=='+'||fu.top=='-'||fu.top=='*'|fu.top=='/'))
                                                 ~~~~~~^~~~~
foo.cc:26:67: error: invalid operands of types '<unresolved overloaded function type>' and 'char' to binary 'operator=='
     while(a[i]!='('&&(fu.top=='+'||fu.top=='-'||fu.top=='*'|fu.top=='/'))
                                                             ~~~~~~^~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。