There are n gas stations in a circle. Station i has gas[i] fuel, and it costs cost[i] to travel to the next station.
Find the starting station index to complete the circuit. Return if impossible.
With gas = [1,2,3,4,5], cost = [3,4,5,1,2]:
- Start at station 3. Tank: 0 + 4 = 4. Cost to next: 1. Tank: 3.
- Station 4: Tank: 3 + 5 = 8. Cost: 2. Tank: 6.
- Station 0: Tank: 6 + 1 = 7. Cost: 3. Tank: 4.
- ... Continue around. Possible!
- Answer: .
Constraints: . Exactly one solution exists if answer is not .