134. 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
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.

Solution

// if a car starts at A and fails at/before B, then any point between A and
// B cannot be a start point

// If the total number of gas is bigger than the total number of cost. There
// must be a solution.
public int canCompleteCircuit(int[] gas, int[] cost) {
int sumGas = 0;
int sumCost = 0;
int start = 0;
int tank = 0;
for (int i = 0; i < gas.length; i++) {
sumGas += gas[i];
sumCost += cost[i];
tank += gas[i] - cost[i];
if (tank < 0) {
start = i + 1;
tank = 0;
}
}
if (sumGas < sumCost) {
return -1;
} else {
return start;
}
}