/***
 * 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 its next station (i+1). You begin the journey with an empty tank at one of the gas stations.
 *
 * Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.
 *
 * Note:
 * The solution is guaranteed to be unique.
 */
public class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int n = gas.length;
        int total = 0;
        int j = -1;
        for (int i = 0, sum = 0; i < n; i++) {
            sum += gas[i] - cost[i];
            total += gas[i] - cost[i];
            if (sum < 0) {
                j = i;
                sum = 0;
            }
        }
        return total >= 0? j + 1 : -1;
   }
}

add GasStation.java