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 N=gas.size();
int sum
;
for(int i=0;i<N;i++) sum[i]=gas[i]-cost[i];

//check
int total=0;
for(int i=0;i<N;i++) total=total+sum[i];
if(total<0) return -1;

//find 1st -
int first=0;
if(sum[first]>=0)
while(sum[first]>=0)
{
first=(first+1)%N;
if(first==0) return 0;
}
else
while(sum[(first-1+N)%N]<0) first=(first-1+N)%N;

int minvalue=0;
int minindex;

int i=first;
do
{
int total=0;
while(sum[i]<0)
{
total=total+sum[i];
i=(i+1)%N;
}
if(total<minvalue)
{
minvalue=total;
minindex=i;
}
while(sum[i]>=0) i=(i+1)%N;
}while(i!=first);
return minindex;
}
};