Gas Station 25 XP Medium

n gas stations in a circle. gas[i] = fuel at station i, cost[i] = fuel to travel to next station. Find the starting station to complete the circuit, or -1.

Greedy: if total gas >= total cost, solution exists. Track running tank; when it goes negative, next station becomes the new start.

💡 Pro tip: Understand this problem deeply — don't just memorize the code. Try explaining the approach out loud as if teaching a friend. If you can explain it simply, you truly understand it!