You should aim for a solution with O(n) time and O(1) space, where n is the size of the input array.

A brute force approach would be to start at each gas station, simulate the process, and return the index where completing the circuit is possible. This would be an O(n^2) time solution. Can you think of a better way? Maybe a greedy approach works.

We can immediately return -1 if sum(gas) < sum(cost), as completing the circuit is impossible due to insufficient gas. Otherwise, a solution always exists because the total gas is sufficient to cover the total cost, meaning there must be a valid starting point that allows completing the circuit.

We start with a variable total to track the gas balance and initialize the result index to 0. As we iterate through the array with index i, we accumulate the difference (gas[i] - cost[i]). If total becomes negative at any index, we reset it to 0 and update the result index to (i + 1).