Method exch

src/common/priority_queue.rs:253–256

(&mut self, i: usize, j: usize)

    }

    fn exch(&mut self, i: usize, j: usize) {
        self.pq.swap(i, j);
        self.qp.swap(self.pq[i] as usize, self.pq[j] as usize);
    }
}