# Gas Station

## Problem

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.

## Example

Given 4 gas stations with gas[i]=[1,1,3,1], and the cost[i]=[2,2,1,1]. The starting gas station's index is 2.

## Note

The solution is guaranteed to be unique.

## Challenge

O(n) time and O(1) extra space

## Code - Java

public class Solution {
/**
* @param gas:  an array of integers
* @param cost: an array of integers
* @return: an integer
*/
public int canCompleteCircuit(int[] gas, int[] cost) {
for (int start = 0; start < gas.length; start++) {
int i = start;
int g = gas[start];
while (true) {
g -= cost[i];
if (g < 0) break;
i = (i + 1) % cost.length;
if (i == start) return i;
g += gas[i];
}
}
return -1;
}
}