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;    }};