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There are `n` gas stations along a circular route, where the amount of gas at station `i` is `gas[i]`. You have a car with an unlimited gas tank and it costs `cost[i]` of gas to travel from station `i` to station `i + 1` (the last station wraps around to station 0). Given two integer arrays `gas` and `cost`, return the starting station index if you can travel around the circuit once in the clockwise direction, or -1 if it is impossible. If a solution exists, it is guaranteed to be unique. For example, given gas = [1,2,3,4,5] and cost = [3,4,5,1,2], starting at station 3 lets you complete the circuit, so return 3.
Example 1
Input: gas = [1,2,3,4,5], cost = [3,4,5,1,2]
Output: 3
Explanation: Starting at station 3: tank 0+4-1=3, station 4: 3+5-2=6, station 0: 6+1-3=4, station 1: 4+2-4=2, station 2: 2+3-5=0. Never negative.
Example 2
Input: gas = [2,3,4], cost = [3,4,3]
Output: -1
Explanation: The total gas (2 + 3 + 4 = 9) is less than the total cost (3 + 4 + 3 = 10). No matter where you start, you run out before finishing the loop, so the answer is -1.
Example 3
Input: gas = [5], cost = [4]
Output: 0
Explanation: The single station gives 5 gas, and returning to it costs 4. Starting at station 0 completes the circuit.
If total gas >= total cost, a solution must exist. To find it: scan once, tracking the current tank. Whenever the tank goes negative, the answer cannot be any station we have passed — reset and try the next station.
Run checks all cases above. Submit evaluates all test cases.