# Minimum number of refueling stops

August 20, 2022

heapProblem URL: Minimum number of refueling stops

We can be greedy and use a max heap to to count the number of refueling stop. First we will append the target to the stations list, where fuel will be 0. We will assing the startFuel as nextStop as it can take us up to the next stop. Then we iterate over the stations, if the next stop is less than the station's position, we will take a value out from the max heap, if our heap is empty, we immediately return -1. Then we check whether the fuel can take us to the next stop, if possible, then we append it to the max heap.

```
class Solution:
def minRefuelStops(self, target: int, startFuel: int, stations: List[List[int]]) -> int:
bag = []
nextStop = startFuel
ans = 0
stations.append([target, 0])
for station in stations:
while station[0] > nextStop:
if not bag:
return -1
nextStop -= heapq.heappop(bag)
ans += 1
if station[0] <= nextStop:
heapq.heappush(bag, -1*station[1])
return ans
```

Time Complexity: `O(n*log(n))`

Space Complexity: `O(n)`