GasStation.java

package greedy;

/**
 * Created by gouthamvidyapradhan on 28/06/2017.
 * There are N gas stations along a circular route, where the amount of gas at station i is gas[i].
 * <p>
 * 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.
 * <p>
 * Return the starting gas station's index if you can travel around the circuit once, otherwise return -1.
 * <p>
 * Note:
 * The solution is guaranteed to be unique.
 * <p>
 * Solution: O(N)
 * If point B cant be reached from point A then all the intermediate points between A and B cant be the starting
 * point. Therefore reset the starting point to the new starting point.
 */
public class GasStation {
    /**
     * Main method
     *
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        int[] gas = {10, 20, 30, 10};
        int[] cost = {5, 30, 10, 10};
        System.out.println(new GasStation().canCompleteCircuit(gas, cost));
    }

    public int canCompleteCircuit(int[] gas, int[] cost) {
        int debt = 0, sum = 0, start = 0;
        for (int i = 0; i < gas.length; i++) {
            sum += gas[i] - cost[i];
            if (sum < 0) {
                debt += sum;
                sum = 0;
                start = i + 1;
            }
        }
        debt += sum;
        return debt >= 0 ? start : -1;
    }

}