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 len = gas.size();
        int ind = 0;
        int remain = 0;
        int hold = 0;
        bool flag = false;
        for(int i=0;i<len;i++){
            int r = gas[i]-cost[i];
            remain += r;

            if(flag == false && r > 0){
                ind = i;
                flag = true;
                hold = hold + remain;
                remain = 0;
            }
            if(remain < 0) flag = false;
        }
        if(hold + remain >= 0) return ind;
        else return -1;
    }
};