GasStation.cpp

#include <iostream>
#include <vector>
using namespace std;

int canCompleteCircuit(vector<int> &gas, vector<int> &cost)
{
    // first of all we need to check whether we've sfficient fuel or not.
    int total_cost = 0, total_fuel = 0, n = cost.size();
    for (int i = 0; i < n; i++)
    {
        total_cost += cost[i];
        total_fuel += gas[i];
    }
    // If the total fuel is lesser than the cost then definitely we can't cover the whole cicular tour.
    if (total_fuel < total_cost)
    {
        return -1;
    }

    // If the total fuel is sufficient enough to cover the circular tour then definitely an answer exists
    int curr_fuel = 0, start = 0; // start with zero fuel.
    for (int i = 0; i < n; i++)
    {
        // If at any point our balance/ current fuel is negative that means we can't come to the i'th petrol pump from the previous pump beacuse our fuel won't allow us to cover such distance.
        // So we'll make the i'th pump as the start point ans proceed. Simultaneously we'll make the current fuel to be 0 as we're starting freshly.
        if (curr_fuel < 0)
        {
            start = i;
            curr_fuel = 0;
        }
        // at any station we'll fill petrol and pay the cost to go to the next station . so current fuel would be the following.
        curr_fuel += (gas[i] - cost[i]);
    }
    return start;
}