View Compilation Error

0_0_1550229_22334.cpp: In function `int main()':
0_0_1550229_22334.cpp:39: error: could not convert `q.std::priority_queue<_Tp, _Sequence, _Compare>::empty [with _Tp = node, _Sequence = std::vector<node, std::allocator<node> >, _Compare = std::less<node>]' to `bool'
0_0_1550229_22334.cpp:39: error: in argument to unary !