CODE 131. Gas Station

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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 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 int canCompleteCircuit(int[] gas, int[] cost) {
	// IMPORTANT: Please reset any member data you declared, as
	// the same Solution instance will be reused for each test case.
	if (null == gas || gas.length <= 0) {
		return -1;
	}
	for (int i = 0; i < gas.length; i++) {
		gas[i] -= cost[i];
	}
	int i = 0;
	int j = 0;
	int sum = 0;
	for (; i < gas.length; i++) {
		if (gas[i] < 0) {
			continue;
		}
		sum = gas[i];
		j = (i + 1) % gas.length;

		while (sum >= 0 && j != i) {
			sum += gas[j];
			j = (j + 1) % gas.length;
		}
		if (j == i) {
			break;
		}
		if (j > i) {
			i = j;
		}
	}

	if (i < gas.length && j == i && sum >= 0) {
		return i;
	} else {
		return -1;
	}
}