/in/foo.cc: In function 'bool sudoku(int, int)':
/in/foo.cc:30:25: error: 'bool sudoku(int, int)' redeclared as different kind of symbol
 bool sudoku(int r, int c) // 回溯
                         ^
/in/foo.cc:6:6: note: previous declaration 'bool sudoku [10][10]'
 bool sudoku[I][I];
      ^~~~~~
/in/foo.cc:41:32: error: 'sudoku' cannot be used as a function
             if (sudoku(r + 1, 0))
                                ^
/in/foo.cc:48:32: error: 'sudoku' cannot be used as a function
             if (sudoku(r, c + 1))
                                ^
/in/foo.cc:63:36: error: 'sudoku' cannot be used as a function
                 if (sudoku(r + 1, 0))
                                    ^
/in/foo.cc:70:36: error: 'sudoku' cannot be used as a function
                 if (sudoku(r, c + 1))
                                    ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:94:16: error: 'sudoku' cannot be used as a function
     sudoku(0, 0);
                ^