121. Gas Station

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.