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