134. 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 sum = 0;
        int total = 0;
        int j = -1;
        int n = gas.size();
        
        for(int i=0;i<n;i++)
        {
            sum += gas[i] - cost[i];
            total += gas[i] - cost[i];
            if(sum<0)
            {
                sum = 0;
                j = i;
            }
        }
        if(total<0)
          return -1;
        return j+1;
    }
};