Gas Station

Source

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

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) {
        int start=0;
        int sum=0;
        int total=0;

        for(int i=0; i<gas.length; i++){
            sum += gas[i]-cost[i];
            total += gas[i]-cost[i];
            if(sum<0){
                start=i+1;
                sum=0;
            }
        }
        if(total<0){
            return -1;
        }
        return start;
    }
}