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.
The solution is guaranteed to be unique.

Solution: ...

class Solution {
public:
int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {
int N = gas.size();
int res = 0, min = gas[0] - cost[0], sum = min;
for (int i = 1; i < N; ++i)
{
sum += gas[i] - cost[i];
if (sum < min) { // not understand it completely
min = sum;
res = i;
}
}
return sum >= 0 ? (res + 1) % N : -1;
}
};