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.

int canCompleteCircuit(vector<int> &gas, vector<int> &cost) {        // Note: The Solution object is instantiated only once.int total = 0;int currentgas = 0; int startpoint = -1;int sz = gas.size();for(int i = 0; i < sz; i++){currentgas += gas[i] - cost[i];total += gas[i] - cost[i];if(currentgas < 0){startpoint = i;currentgas = 0;}}return total >= 0 ? startpoint+1 : -1;    }