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) {
        int n = gas.size();
        vector<int> a;
        int sum = 0;
        for(int i = 0; i < n; i++)
        {
            a.push_back(gas[i] - cost[i]);
            sum += gas[i] - cost[i];
        }
        if(sum < 0)return -1;
        
        int i = 0;
        sum = 0;
        int j = 0;
        while(i < n - 1)
        {
            sum += a[i];
            if(sum < 0)
            {
                sum = 0;
                j = i+1;
            }
            i++;
        }
        
        return j;
    }
};