Minimum Number of Refills for a Car Journey

You are driving to a destination that is a certain distance away, and your car has a limited fuel tank capacity. Along the way, there are gas stations at specific distances from your starting point. The task is to determine the **minimum number of refills** you will need to reach the destination, if possible, based on the given car fuel capacity and the locations of the gas stations.

### Inputs:

1. **Distance to destination** (`dist`): The total distance you need to travel.

2. **Fuel tank capacity** (`miles`): The maximum number of miles your car can travel on a full tank.

3. **Number of gas stations** (`n`): The number of gas stations along the route.

4. **Gas station locations** (`gas_stations`): A list of distances where gas stations are located from your starting point.

### Objective:

Determine the minimum number of refills required to reach the destination. If it’s not possible to reach the destination, return `-1`.

### Solution

1. **Initial Setup:**

   - You start with a full tank of gas, so the car can travel `miles` distance before needing a refill.

   - The variable `num_refill` keeps track of the total number of refills.

   - `curr_refill` tracks your current position at a gas station, and `limit` keeps track of how far you can travel with the current fuel level.

2. **Loop Until the Destination is Reachable:**

   - While the destination is beyond your current limit (i.e., `limit < dist`), you need to check whether you can reach the destination or the next gas station.

   - If you can't reach the next gas station (or there are no more stations), return `-1` to indicate that the trip is impossible.

3. **Find the Furthest Reachable Gas Station:**

   - The algorithm moves to the furthest reachable gas station that is within the current fuel limit.

   - It iterates through the list of gas stations and moves `curr_refill` to the furthest station within range.

   

4. **Refill at the Furthest Station:**

   - After finding the furthest gas station within reach, refill the tank, increase the refill count, and update the `limit` to reflect how far you can now travel after refilling.

5. **Continue Until the Destination is Reachable:**

   - The loop continues, finding the next furthest reachable station and refilling until you can reach the destination.

6. **Return the Number of Refills:**

   - If you reach the destination, return the number of refills made during the journey.

### Step-by-Step Breakdown

1. **Check Feasibility**:

   - Start with a full tank, and as long as the destination cannot be reached with the current fuel (`limit < dist`), look for gas stations within range.

2. **Handle Edge Cases**:

   - If the gas station is too far away from the current limit or there are no more gas stations within reach, return `-1` to indicate that the destination is unreachable.

3. **Optimize Refills**:

   - Always stop at the furthest reachable gas station to minimize the number of refills. This prevents unnecessary stops at closer stations.

### Example Walkthrough:

#### Test Case 1:

- **Input:** `car_fueling(950, 400, 4, [200, 375, 550, 750])`

  - **Explanation:**

    - Start with a full tank (can travel 400 miles).

    - Reach the first station at 200 miles, but continue to the furthest station at 375 miles.

    - Refill at 375 miles and continue.

    - Reach the next station at 550 miles but continue to the furthest one at 750 miles.

    - Refill at 750 miles and then reach the destination at 950 miles.

  - **Result:** 2 refills are needed.

#### Test Case 2:

- **Input:** `car_fueling(10, 3, 4, [1, 2, 5, 9])`

  - **Explanation:**

    - Start with a full tank (can travel 3 miles).

    - The first station is at 1 mile, but you cannot reach the next station at 5 miles with the current fuel capacity.

  - **Result:** The trip is impossible, so return `-1`.