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;
}
};