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;
}
};