foo.cc: In function 'void solve()':
foo.cc:40:31: warning: array subscript has type 'char' [-Wchar-subscripts]
             if (ptrval[new_val])
                               ^
foo.cc:42:27: warning: array subscript has type 'char' [-Wchar-subscripts]
             ptrval[new_val] = true;
                           ^
foo.cc:65:35: warning: array subscript has type 'char' [-Wchar-subscripts]
             ptrval[val_stack.top()] = false;
                                   ^

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