Gas Station

Question:

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.

Solution:

 1 class Solution {
 2 public:
 3     int canCompleteCircuit(vector<int>& gas, vector<int>& cost) {
 4     int sum=0;
 5     int index;
 6     bool flag=false;
 7     int count=0;
 8     for(int i=0;count<=gas.size();i++)
 9     {
10         if(flag==false)
11             index=i;
12         flag=true;
13         if(i<gas.size())
14             sum+=gas[i]-cost[i];
15         else
16             sum+=gas[i-gas.size()]-cost[i-gas.size()];
17         count++;
18         if(sum<0)
19         {
20             flag=false;
21             sum=0;
22             count=0;
23         }
24         if(count==gas.size()) return index;
25         if(i>=2*gas.size()-1) {return -1;}
26     }
27     }
28 };