gas-station leetcode C++

There are N gas stations along a circular route, where the amount of gas at station i isgas[i].

You have a car with an unlimited gas tank and it costscost[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.

C++

class Solution {
public:
    int canCompleteCircuit(vector &gas, vector &cost) {
        int total = 0, sum = 0;
        int index = -1;
        for (int i = 0; i < gas.size(); i++) {
            sum += gas[i] - cost[i];        
            total += gas[i] - cost[i];  
            if (sum < 0) {
                sum = 0;
                index = i;    
            }
        }
        return total >= 0 ? index + 1 : -1;
    }
};