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.

int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
int tank=0, sum=0, pos=0;
for(int i=0;i<gas.size();i++)
{
tank += gas[i]-cost[i];
sum += gas[i]-cost[i];
if(tank<0)
{
tank=0;
pos=i+1;
}
}
if(sum<0)
return -1;
else
return pos%gas.size();
}

execution time: 56ms