def run(self):
pq = PriorityQueue()
for q in self.queues:
t = q.reset()
pq.push(t, q)
while not pq.empty():
t = max(t, pq.key())
q = pq.value()
pq.pop()
next_t = q.process(t)
if next_t >= 0:
pq.push(next_t, q)
return t
