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.

Note:
The solution is guaranteed to be unique.

class Solution {
public:
    int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
        int n = gas.size();
        if (n < 1)
        {
            return -1;
        }

        int i = 0;
        int j = n;
        int sum = 0;
        int cur = 0;
        while (i < j)
        {
            sum += gas[cur] - cost[cur];
            if (sum >= 0)
            {
                i++;
                cur = i;
            }
            else
            {
                j--;
                cur = j;
            }
        }

        if (sum >= 0)
        {
            return j%n;
        }

        return -1;
    }
};